MID TEST ANSWERS OF ORGANIC CHEMISTRY 1

COURSE : ORGANIC CHEMISTRY I

CREDIT   : 3 SKS

DAY/ TIME      : TUESDAY, NOVEMBER 13TH 2012-11-13

LECTURE        : Dr. Syamsurizal

NAME              : MUHAMMAD HAQIQI

NIM          : RSA1C111O12

1 A. Explain how the concept of organic compounds from petroleum can be used as a fuel for vehicles such as car , motor bike , including aricraft .

 B Explain it how the idea of chemical compounds of petroleum can be used to make clothing and plastic and materials needs of other human lives .

2. Explain why the hydrocarbon asymetrical or chiral have a variety of benefit for human being . and describe how does it the chiral centers can be formed .

 3.  When ethylene gas produced from a ripe fruit can be used to ripe other fruits that are still unripe . how do you idea when the gas is used as fuel gas like methane gas.

  4. Aromatic compounds are marked by ease of adjacent electrons conjugated . please explain why an unsaturated compund which highly conjugated but is not aromatic ?

Answer

1.      A.

 We Know  Petroleum or crude oil is a naturally occurring flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. A fossil fuel, it is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock and undergo intense heat and pressure. Petroleum is a mixture of a very large number of different hydrocarbons; the most commonly found molecules are alkanes (linear or branched),cycloalkanes, aromatic hydrocarbons, or more complicated chemicals like asphaltenes. Each petroleum variety has a unique mix of molecules, which define its physical and chemical properties, like color and viscosity.

The alkanes, also known as paraffins, are saturated hydrocarbons with straight or branched chains which contain only carbon and hydrogen and have the general formula C_nH_2n+2. They generally have from 5 to 40 carbon atoms per molecule, although trace amounts of shorter or longer molecules may be present in the mixture.

The alkanes from pentane (C₅H₁₂) to octane (C₈H₁₈) are refined into petrol, the ones from nonane (C₉H₂₀) to hexadecane (C₁₆H₃₄) into diesel fuel,kerosene and jet fuel. Alkanes with more than 16 carbon atoms can be refined into fuel oil and lubricating oil. At the heavier end of the range, paraffin wax is an alkane with approximately 25 carbon atoms, while asphalt has 35 and up, although these are usually cracked by modern refineries into more valuable products. The shortest molecules, those with four or fewer carbon atoms, are in a gaseous state at room temperature. They are the petroleum gases. Depending on demand and the cost of recovery, these gases are either flared off, sold as liquified petroleum gas under pressure, or used to power the refinery's own burners. During the winter, butane (C₄H₁₀), is blended into the petrol pool at high rates, because its high vapor pressure assists with cold starts. Liquified under pressure slightly above atmospheric, it is best known for powering cigarette lighters, but it is also a main fuel source for many developing countries. Propane can be liquified under modest pressure, and is consumed for just about every application relying on petroleum for energy, from cooking to heating to transportation.

The cycloalkanes, also known as naphthenes, are saturated hydrocarbons which have one or more carbon rings to which hydrogen atoms are attached according to the formula C_nH_2n. Cycloalkanes have similar properties to alkanes but have higher boiling points.

The aromatic hydrocarbons are unsaturated hydrocarbons which have one or more planar six-carbon rings called benzene rings, to which hydrogen atoms are attached with the formula C_nH_n. They tend to burn with a sooty flame, and many have a sweet aroma. Some are carcinogenic.

These different molecules are separated by fractional distillation at an oil refinery to produce petrol, jet fuel, kerosene, and other hydrocarbons. For example, 2,2,4-trimethylpentane (isooctane), widely used in petrol, has a chemical formula of C₈H₁₈ and it reacts with oxygen exothermically:^[18]

2 C₈H_18(l) + 25 O_2(g) → 16 CO_2(g) + 18 H₂O_(g) (ΔH = −10.86 MJ/mol of octane)

The amount of various molecules in an oil sample can be determined in laboratory. The molecules are typically extracted in a solvent, then separated in a gas chromatograph, and finally determined with a suitable detector, such as a flame ionization detector or a mass spectrometer.^[19] Due to the large number of co-eluted hydrocarbons within oil, many cannot be resolved by traditional gas chromatography and typically appear as a hump in the chromatogram. This unresolved complex mixture (UCM) of hydrocarbons is particularly apparent when analysing weathered oils and extracts from tissues of organisms exposed to oil.

Incomplete combustion of petroleum or petrol results in production of toxic byproducts. Too little oxygen results in carbon monoxide. Due to the high temperatures and high pressures involved, exhaust gases from petrol combustion in car engines usually include nitrogen oxides which are responsible for creation of photochemical smog.

Petroleum Can be formed as a fuel for vehicles is The range of carbon chain: C6 to C11

Boiling Route: 50 to 85 ° C

Gasoline obtained from the distillation of petroleum caused many knock (knocking). The knock caused the "self ignition", which combustion occurs too quickly before the piston is in the right position. The more beats, the less efficient use of fuel and can damage the engine therefore only burn gasoline in the vapor phase, the gasoline must be vaporized in the carburetor before being burned in the engine cylinder. The energy generated from the combustion of gasoline is converted into motion through the following steps.

Burning gasoline is desired that produces a smooth impetus to the decline of the piston. It depends on the timeliness of combustion so that the amount of energy transferred to a maximum piston. Timeliness combustion depending on the type of hydrocarbon chains which in turn will determine the quality of gasoline.

Octane Numbers

Inside the machine, a mixture of air and fuel (a gas) is pressed by the piston to a very small volume and then set on fire by sparks produced plugs. Because they magnitude of this pressure, the air fuel mixture can ignite spontaneously before the spark from the spark plug out. Octane number of the gasoline to provide information to us about how much pressure the bias given before the gasoline ignites spontaneously. If the gas mixture is ignited due to the high pressure (and not because of the spark plugs), there will be a knock or knock in the engine. This will cause engine knocking easily damaged, so this should be avoided.

Name derived from octane octane (C8), because of all the molecules making up petrol, octane has the best compression properties, octane can be compressed to a small volume without experiencing spontaneous combustion, not as happens in heptane, for example, which can ignite spontaneously although freshly pressed slightly .

Gasoline with 87 octane, the fuel consists of 87% octane and 13% heptane (or a mixture of other molecules). Gasoline will ignite spontaneously at a certain compression level numbers given that only meant that the vehicle's engine has a compression ratio that does not exceed that number.

Gasoline is a petroleum distillate having a low octane number (<60), because it consists mainly straight-chain alkanes. Low octane number can be increased by adding anti-knock additive substance that processes the conversion of straight-chain alkanes into branched chain. Anti-knock substances that have been used include:

a. Tetra Ethyl Lead (TEL)

Molecular formula Pb (C2H5) 4. TEL was banned for use as current use in gasoline combustion can produce lead oxide (PbO) attached to the engine components. Agar (PbO) does not stick to the use of TEL (65%) was added to 1,2-dibromo ethane and 1,2-dichloro ethane that turns into PbBr2 Pb (volatile) coming out of the exhaust. These substances can pollute the air and if it enters the body will result in anemia, headache, and when in high levels can cause death.

b. Ethyl Tertiary Butyl Ether (ETBE)

Molecular formula CH3O (C2H5) 3

c. Tertiary amyl methyl ether (TAME)

Molecular formula CH 3 O (CH3) C2H5

d. Methyl Tertiary Butyl Ether (MTBE)

Molecular formula CH3O (CH3) 3

Additive most widely used to date. However, its use is also limited because of toxic and cancer-causing. Premix gasoline using a mixture of MTBE and TEL.

Exhaust fumes may produce CO, CO2, SO2 and NOx. Gas COsangat dangerous if inhaled too much can lead to death, because disturb the binding of oxygen by hemoglobin.

1.b Establishment of clothing, plastics and other materials (naphtha) that humans need of petroleum refining occurs in the fourth fraction of the processing of the first distillation of petroleum with a rise in temperature of less than 200 degrees Celsius. In this route, naphtha (gasoline by weight) will melt and come out to the shelter naphtha. Naphtha is a mixture of alkanes with chain C9H20-C12H26

In subsequent processing to get results without material adverse human made chemical processes such as cracking, polymerase, and reform.

The trick can be implemented as follows:

• Thermal Cracking, ie cracking process using high temperature and pressure only.

• Cracking catalytic, ie cracking process using heat and catalysts to convert distillate that has a high boiling point into gasoline and karosin. This process will also produce butane and other gases.

• Cracking with hydrogen (hydro-cracking), ie cracking process which is a combination of thermal and catalytic cracking to "inject" hydrogen molecule unsaturated hydrocarbon fraction. In this way, it can be produced from petroleum LPG, naphtha, karosin, jet fuel, and diesel. Amount obtained will be more and better quality than the thermal cracking process or catalytic cracking alone. In addition, the residual amount will be reduced

Polymerization is a merger of two or more molecules to form single molecules called polymers. Polymerization goal was to combine hydrocarbon molecules in a gas (ethylene, propene) to a compound of light naphtha.

Reformation

This process can be a mild thermal cracking of naphtha to obtain a more volatile products such as olefins with a higher octane number. In addition, it can also be the catalytic conversion of naphtha components untukmenghasilkan aromatic with a higher octane number

2.       

Atom c kiral

Hydrocarbons chiral forms are beneficial to humans because hydrocarbons have optical isomers. Optical isomers are isomers caused by different directions of rotating the plane of polarization of light. Active optical properties of a compound due to the asymmetric C atom (ie C atoms that bind four different atoms or groups)

* Dekstro (d) turn to the right (clockwise)

* Levo (l) turn to the left (counter clockwise)

Chiral compounds are formed because it has an asymmetric C atom, the C atom that binds to 4 atoms / groups of different atoms, has a pair of optical isomers. This is caused by the ability of these compounds rotate plane polarized light. Rotation to the left labeled l (levo = levus) and rotation to the right mark d (dekstro = dekster). For example, 2-hydroxy propanoic acid, CH3CHOHCOOH. Atom C no. (2) binding H, OH, CH3, and COOH. C atom is called asymmetric or chiral. Couple isomer (d) and (l) is called enantiomer pair. This isomer pair one being the mirror image of the other.

3.       

Ethylene (ethene H2C = H2) with molecular weights 28.0536 an olefinic hydrocarbons are the lightest, colorless liquid, flammable gas, smelling sweet. These compounds contained in natural gas, petroleum dirty, or other fossil fuel deposits. However, ethylene can also be obtained in large quantities from a variety of thermal and catalytic processes with high temperature fractions of natural gas and petroleum as a raw material. Ethylene glycol or called Monoetilen Glycol, resulting from the reaction of ethylene oxide with water, an antifreeze agent used on the machines, also used for the production of raw materials polietilenterephthalate (PET) and as a liquid heat exchanger. Ethylene glycol is an organic compound that can lower the freezing point of the solvent to disrupt the formation of ice crystals solvent. Ethylene Glycol is a liquid saturated, colorless, odorless, sweet taste, and dissolve completely in water. Commercially, ethylene glycol in Indonesia are used as raw material for polyester industry (textiles) of 97.34%. Ethylene Glycol (1,2-etandiol, HOCH2CH2OH) by Mr 62.07 is a simple diol compounds. Ethylene Glycol in the form of a colorless liquid with a sweet aroma. The compound is hygroscopic and dissolve completely in various polar solvents such as water, alcohols, glycol ethers, and acetone. Slightly soluble in nonpolar solvents, such as benzene, toluene, dikloroetan, and chloroform. Ethylene glycol is hard crystallized when cold, he shaped compound that is very thick (viscous)

Reaction etilene with oxygen

ethylene gas produced from ripe fruit can ripen grapes.

To my knowledge we can use ethylene gas as antifreeze in cars. But I think if it is used as fuel could have happened when we reaksikan with oxygen. But the quality of the fuel produced is not necessarily better than methane. Gases such as carbon dioxide produced can be so much more than the amount of methane and ethylene gas heat generated is less than that of methane. Unless processed again to produce more heat. But it has been proven fuel methane gas is more environmentally friendly than other hydrocarbon materials

4.      
Based on the arrangement of carbon atoms in the molecule, carbon compounds are divided into two major categories, namely compound aliphatic and cyclic compounds. Aliphatic hydrocarbons are carbon compounds chain opens its C and C it allows branched chain. Based on the amount of the bond, aliphatic hydrocarbons, aliphatic compounds are divided into saturated and unsaturated. 

- The compound is a saturated aliphatic C chain aliphatic compounds it contains only single bonds only. This group is called alkanes. 

Unsaturated aliphatic compounds are aliphatic compounds, varying chain C double bond or triple. If you have duplicate named alkenes and alkynes have triple called. In unsaturated compounds (-C = O), the transition to the low-energy non-bonding involves electrons to oxygen, one of it was promoted to the p * orbital which is relatively low. However, the transition from n to p *, called "forbidden" or including a ban on the transition, it is associated with a low intensity. Two others, namely the transition from n to s * and from p to s *. Both give strong absorption, but involves high energy. The most noticeable absorption intensity for ketone compounds are electron transition p to p *

Acid and Bases Organic

An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are relatively stronger acids. Alcohols, with –OH, can act as acids but they are usually very weak. The relative stability of the conjugate base of the acid determines its acidity. Other groups can also confer acidity, usually weakly: the thiol group –SH, the enol group, and the phenol group. In biological systems, organic compounds containing these groups are generally referred to as organic acids.

A few common examples include:
Lactic acid
Acetic acid
Formic acid
Citric acid
Oxalic acid
Uric acid

Characteristics

In general, organic acids are weak acids and do not dissociate completely in water, whereas the strong mineral acids do. Lower molecular mass organic acids such as formic and lactic acids are miscible in water, but higher molecular mass organic acids, such as benzoic acid, are insoluble in molecular (neutral) form.

On the other hand, most organic acids are very soluble in organic solvents. p-Toluenesulfonic acid is a comparatively strong acid used in organic chemistry often because it is able to dissolve in the organic reaction solvent.

Exceptions to these solubility characteristics exist in the presence of other substituents that affect the polarity of the compound.

Applications

Simple organic acids like formic or acetic acids are used for oil and gas well stimulation treatments. These organic acids are much less reactive with metals than are strong mineral acids like hydrochloric acid (HCl) or mixtures of HCl and hydrofluoric acid (HF). For this reason, organic acids are used at high temperatures or when long contact times between acid and pipe are needed.[citation needed]

The conjugate bases of organic acids such as citrate and lactate are often used in biologically-compatible buffer solutions.

Citric and oxalic acids are used as rust removal. As acids, they can dissolve the iron oxides, but without damaging the base metal as do stronger mineral acids. In the dissociated form, they may be able to chelate the metal ions, helping to speed removal.[citation needed]

Biological systems create many and more complex organic acids such as L-lactic, citric, and D-glucuronic acids that contain hydroxyl or carboxyl groups. Human blood and urine contain these plus organic acid degradation products of amino acids, neurotransmitters, and intestinal bacterial action on food components. Examples of these categories are alpha-ketoisocaproic, vanilmandelic, and D-lactic acids, derived from catabolism of L-leucine and epinephrine (adrenaline) by human tissues and catabolism of dietary carbohydrate by intestinal bacteria, respectively.

Application in food

                                  The general structure of a few weak organic acids. From left to right: phenol, enol, alcohol, thiol. The acidic hydrogen in each molecule is colored red.

                                  The general structure of a few organic acids. From left to right: carboxylic acid, sulfonic acid. The acidic hydrogen in each molecule is colored red.

Organic acids are used in food preservation because of their effects on bacteria. The key basic principle on the mode of action of organic acids on bacteria is that non-dissociated (non-ionized) organic acids can penetrate the bacteria cell wall and disrupt the normal physiology of certain types of bacteria that we call pH-sensitive, meaning that they cannot tolerate a wide internal and external pH gradient. Among those bacteria are Escherichia coli, Salmonella spp., C. perfringens, Listeria monocytogenes, and Campylobacter species.

Upon passive diffusion of organic acids into the bacteria, where the pH is near or above neutrality, the acids will dissociate and lower the bacteria internal pH, leading to situations that will impair or stop the growth of bacteria. On the other hand, the anionic part of the organic acids that cannot escape the bacteria in its dissociated form will accumulate within the bacteria and disrupt many metabolic functions, leading to osmotic pressure increase, incompatible with the survival of the bacteria.

It has been well demonstrated that the state of the organic acids (undissociated or dissociated) is extremely important to define their capacity to inhibit the growth of bacteria, compared to undissociated acids.

Lactic acid and its salts sodium lactate and potassium lactate are widely used as antimicrobials in food products, in particular, meat and poultry such as ham and sausages.
Application in nutrition and animal feeds

Organic acids have been used successfully in pig production for more than 25 years. Although less research has been done in poultry, organic acids have also been found to be effective in poultry production.

Organic acids (C1–C7) are widely distributed in nature as normal constituents of plants or animal tissues. They are also formed through microbial fermentation of carbohydrates mainly in the large intestine. They are sometimes found in their sodium, potassium, or calcium salts, or even stronger double salts.

Organic acids added to feeds should be protected to avoid their dissociation in the crop and in the intestine (high pH segments) and reach far into the gastrointestinal tract, where the bulk of the bacteria population is located.

From the use of organic acids in poultry and pigs, one can expect an improvement in performance similar to or better than that of antibiotic growth promoters, without the public health concern, a preventive effect on the intestinal problems like necrotic enteritis in chickens and Escherichia coli infection in young pigs. Also one can expect a reduction of the carrier state for Salmonella species and Campylobacter species.

An organic base is an organic compound which acts as a base. Organic bases are usually, but not always, proton acceptors. They usually contain nitrogen atoms, which can easily be protonated. Amines and nitrogen-containing heterocyclic compounds are organic bases. Examples include:
pyridine
methyl amine
imidazole
benzimidazole
histidine
phosphazene bases
Hydroxides of some organic cations
[edit]
Factors affecting alkalinity

While all organic bases are considered to be weak, many factors can affect the alkalinity of the compounds. One such factor is the inductive effect. A simple explanation of the term would state that electropositive atoms (such as carbon groups) attached in close proximity to the potential proton acceptor have an "electron-releasing" effect, such that the positive charge acquired by the proton acceptor is distributed over other adjacent atoms in the chain. The converse is also possible as alleviation of alkalinity: electronegative atoms or species (such as fluorine or the nitro group) will have an "electron-withdrawal" effect and thereby reduce the basicity. To this end, trimethylamine is a more potent base than merely ammonia, due to the inductive effect of the methyl groups allowing the nitrogen atom to more readily accept a proton and become a cation being much greater than that of the hydrogen atoms.[citation needed] In guanidines, the protonated form (guanidinium) has three resonance structures, giving it increased stability and making guanadines stronger bases.

Phosphazene bases also contain phosphorus and are, in general, more alkaline than standard amines and nitrogen-based heterocyclics. Protonation takes place at the nitrogen atom, not the phosphorus atom to which the nitrogen is double-bonded.
Hydroxide donors

Some organic bases, such as tetramethylammonium hydroxide, tetrabutylammonium hydroxide, or choline hydroxide are hydroxide donors rather than proton acceptors like the above compounds. However, they are not always stable. Choline hydroxide, for example, is metastable and slowly breaks down to release trimethylamine.

Protein as Palat trasnportasi / carrier and lipid of life

Used mainly for transporting large materials across the cell membrane.  There are specific “protein channels” for specific molecules.




Transport protein is a protein that can bind and carry distinctive molecules or ions from one organ to another organ. Examples are easy to transport protein myoglobin to store and distribute oxygen to the muscles, consider Figure 14:28.

Figure 14:28. Myoglobin, which distributes oxygen to the muscles

Hemoglobin is also a transport protein found in red blood cells. Hemoglobin can bind oxygen when the blood through the lungs. Oxygen was taken and released on peripheral tissues that can be used to oxidize nutrients (food) into energy. In blood plasma there is a lipoprotein that serves to transport lipids from the liver to the organ. Other transport proteins present in the cell membrane serves to bring some molecules such as glucose, amino acids and other nutrients through the membrane into the cell.

 Both passive and active transport mediated by the help of a transmembrane protein that acts as a transporter. shows two major classes of transport proteins: carrier proteins and protein channels. For the most part, the carrier protein while the protein mediates the active transport channels mediate passive transport. Protein carrier makes a hole in the lipid bilayer to undergo conformational changes on the binding of molecules. Proteins form pores hydrophilic channel across the lipid bilayer. When open, these pores allow certain molecules to pass through. There is one other class of transport proteins called ionophores. It is a small, hydrophobic protein that increases the permeability of the double layer for a particular ion.
CARRIER PROTEINS move large molecules into or out of the cell down their concentration gradient. Different carrier proteins facilitated the diffusion of different molecules.

1. First, a large molecule attaches to a carrier protein in the membrane. 
2. Then, the protein changes shape. 
3. This releases the molecule on the opposite side of the membrane. 

PROTEIN CHANNELS form pores in the membrane for charged particles to diffuse through (down their concentration gradient). Different protein channels facilitate the diffusion of different charged particles.



☞ ACTIVE TRANSPORT moves substances AGAINST a concentration gradient

Active transport uses ATP energy to move molecules and ions across plasma membranes, against a concentration gradient.

Carrier proteins:
- A molecule attaches to the carrier protein
- The protein changes shape
- This moves the molecule across the membrane
- ATP energy is used to move the solute against its concentration gradient

Co-transporters are a type of carrier protein:
- They bind two molecules at a time
- The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient, essentially sucking it into the black hole that is the other side of the membrane

LIPID 
Lipid refers to the group of aliphatic hydrocarbons nonpolar and hydrophobic. Because nonpolar, lipid insoluble in polar solvents such as water, but soluble in nonpolar solvents, such as alcohol, ether or chloroform. The most important biological functions of these lipids to store energy, as structural components of cell membranes and as signaling molecules.

Lipids are organic compounds derived from the dehydrogenation of hydrocarbons endotermal series. Lipids are amfifilik, meaning that lipids are able to form structures such as vesicles, liposomes, or other membranes in a wet environment. Biological lipids wholly or partly derived from two types subsatuan or "building blocks" of biochemistry: ketoasil group and isoprene groups. [4] Using this approach, lipids may be divided into eight categories: [5] fatty acyl, gliserolipid, gliserofosfolipid, sfingolipid , sakarolipid, and polyketides (derived from condensation subsatuan ketoasil), and sterol lipids and lipid prenol (derived from condensation of isoprene subsatuan).

Although the term lipid is sometimes used as a synonym of fat. Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids, as well as sterol-containing metabolites such as cholesterol. [6] Although humans and mammals have the metabolism to break down and form a lipid, some lipids can not be generated in this way and must be obtained through food.

Fatty acids

Fatty acids, or fatty acid residues when they form part of a lipid, are a diverse group of molecules synthesized by chain-elongation of an acetyl-CoA primer with malonyl-CoA or methylmalonyl-CoA groups in a process called fatty acid synthesis.^[7][8] They are made of a hydrocarbon chain that terminates with a carboxylic acid group; this arrangement confers the molecule with a polar, hydrophilic end, and a nonpolar, hydrophobic end that is insoluble in water. The fatty acid structure is one of the most fundamental categories of biological lipids, and is commonly used as a building-block of more structurally complex lipids.^[9] The carbon chain, typically between four and 24 carbons long,^[10] may be saturated or unsaturated, and may be attached to functional groups containing oxygen, halogens, nitrogen, and sulfur. Where a double bond exists, there is the possibility of either a cis or a trans geometric isomerism, which significantly affects the molecule's molecular configuration. Cis-double bonds cause the fatty acid chain to bend, an effect that is more pronounced the more double bonds there are in a chain. This in turn plays an important role in the structure and function of cell membranes.^[11] Most naturally occurring fatty acids are of the cis configuration, although the trans form does exist in some natural and partially hydrogenated fats and oils.^[12]
Examples of biologically important fatty acids are the eicosanoids, derived primarily from arachidonic acid and eicosapentaenoic acid, that include prostaglandins, leukotrienes, and thromboxanes. Docosahexaenoic acid is also important in biological systems, particularly with respect to sight.^[13][14] Other major lipid classes in the fatty acid category are the fatty esters and fatty amides. Fatty esters include important biochemical intermediates such as wax esters, fatty acid thioester coenzyme A derivatives, fatty acid thioester ACP derivatives and fatty acid carnitines. The fatty amides include N-acyl ethanolamines, such as the cannabinoid neurotransmitter anandamide.^[15]

Glycerolipids

Glycerolipids are composed mainly of mono-, di-, and tri-substituted glycerols,^[16] the most well-known being the fatty acid triesters of glycerol, called triglycerides. The word triacylglycerol is sometimes used synonymously with triglyceride, however this is misleading with respect to these compounds as they contain no hydroxyl group. In these compounds, the three hydroxyl groups of glycerol are each esterified, typically by different fatty acids. Because they function as an energy store, these lipids comprise the bulk of storage fat in animal tissues. The hydrolysis of the ester bonds of triglycerides and the release of glycerol and fatty acids from adipose tissue are the initial steps in metabolising fat.^[17]
Additional subclasses of glycerolipids are represented by glycosylglycerols, which are characterized by the presence of one or more sugar residues attached to glycerol via a glycosidic linkage. Examples of structures in this category are the digalactosyldiacylglycerols found in plant membranes^[18] and seminolipid from mammalian sperm cells.^[19]

Glycerophospholipids

Glycerophospholipids, usually referred to as phospholipids, are ubiquitous in nature and are key components of the lipid bilayer of cells,^[20] as well as being involved in metabolism and cell signaling
.^[21] Neural tissue (including the brain) contains relatively high amounts of glycerophospholipids, and alterations in their composition has been implicated in various neurological disorders.^[22] Glycerophospholipids may be subdivided into distinct classes, based on the nature of the polar headgroup at the sn-3 position of the glycerol backbone in eukaryotes and eubacteria, or the sn-1 position in the case of archaebacteria.^[23]
  
Examples of glycerophospholipids found in biological membranes are phosphatidylcholine (also known as PC, GPCho or lecithin), phosphatidylethanolamine (PE or GPEtn) and phosphatidylserine (PS or GPSer). In addition to serving as a primary component of cellular membranes and binding sites for intra- and intercellular proteins, some glycerophospholipids in eukaryotic cells, such as phosphatidylinositols and phosphatidic acids are either precursors of or, themselves, membrane-derived second messengers.^[24] Typically, one or both of these hydroxyl groups are acylated with long-chain fatty acids, but there are also alkyl-linked and 1Z-alkenyl-linked (plasmalogen) glycerophospholipids, as well as dialkylether variants in archaebacteria.^[25]

Sphingolipids

Sphingolipids are a complicated family of compounds^[26] that share a common structural feature, a sphingoid base backbone that is synthesized de novo from the amino acid serine and a long-chain fatty acyl CoA, then converted into ceramides, phosphosphingolipids, glycosphingolipids and other compounds. The major sphingoid base of mammals is commonly referred to as sphingosine. Ceramides (N-acyl-sphingoid bases) are a major subclass of sphingoid base derivatives with an amide-linked fatty acid. The fatty acids are typically saturated or mono-unsaturated with chain lengths from 16 to 26 carbon atoms.^[27]

Sphingomyelin^[3]

The major phosphosphingolipids of mammals are sphingomyelins (ceramide phosphocholines),^[28] whereas insects contain mainly ceramide phosphoethanolamines^[29] and fungi have phytoceramide phosphoinositols and mannose-containing headgroups.^[30] The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked via a glycosidic bond to the sphingoid base. Examples of these are the simple and complex glycosphingolipids such as cerebrosides and gangliosides.

Sterol lipids

Sterol lipids, such as cholesterol and its derivatives, are an important component of membrane lipids,^[31] along with the glycerophospholipids and sphingomyelins. The steroids, all derived from the same fused four-ring core structure, have different biological roles as hormones and signaling molecules. The eighteen-carbon (C18) steroids include the estrogen family whereas the C19 steroids comprise the androgens such as testosterone and androsterone. The C21 subclass includes the progestogens as well as the glucocorticoids and mineralocorticoids.^[32] The secosteroids, comprising various forms of vitamin D, are characterized by cleavage of the B ring of the core structure.^[33] Other examples of sterols are the bile acids and their conjugates,^[34] which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the phytosterols, such as β-sitosterol, stigmasterol, and brassicasterol; the latter compound is also used as a biomarker for algal growth.^[35] The predominant sterol in fungal cell membranes is ergosterol.^[36]

Prenol lipids

Prenol lipids are synthesized from the five-carbon-unit precursors isopentenyl diphosphate and dimethylallyl diphosphate that are produced mainly via the mevalonic acid (MVA) pathway.^[37] The simple isoprenoids (linear alcohols, diphosphates, etc.) are formed by the successive addition of C5 units, and are classified according to number of these terpene units. Structures containing greater than 40 carbons are known as polyterpenes. Carotenoids are important simple isoprenoids that function as antioxidants and as precursors of vitamin A.^[38] Another biologically important class of molecules is exemplified by the quinones and hydroquinones, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.^[39] Vitamin E and vitamin K, as well as the ubiquinones, are examples of this class. Prokaryotes synthesize polyprenols (called bactoprenols) in which the terminal isoprenoid unit attached to oxygen remains unsaturated, whereas in animal polyprenols (dolichols) the terminal isoprenoid is reduced.^[40]

Saccharolipids

Structure of the saccharolipid Kdo₂-Lipid A.^[41] Glucosamine residues in blue, Kdo residues in red, acyl chains in black and phosphate groups in green.

Saccharolipids describe compounds in which fatty acids are linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a monosaccharide substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids. The most familiar saccharolipids are the acylated glucosamine precursors of the Lipid A component of the lipopolysaccharides in Gram-negative bacteria. Typical lipid A molecules are disaccharides of glucosamine, which are derivatized with as many as seven fatty-acyl chains. The minimal lipopolysaccharide required for growth in E. coli is Kdo₂-Lipid A, a hexa-acylated disaccharide of glucosamine that is glycosylated with two 3-deoxy-D-manno-octulosonic acid (Kdo) residues.^[41]

Polyketides

Polyketides are synthesized by polymerization of acetyl and propionyl subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the fatty acid synthases. They comprise a large number of secondary metabolites and natural products from animal, plant, bacterial, fungal and marine sources, and have great structural diversity.^[42][43] Many polyketides are cyclic molecules whose backbones are often further modified by glycosylation, methylation, hydroxylation, oxidation, and/or other processes. Many commonly used anti-microbial, anti-parasitic, and anti-cancer agents are polyketides or polyketide derivatives, such as erythromycins, tetracyclines, avermectins, and antitumor epothilones.^[44]