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Important functional groups

 

Important functional groups



Functional groups
If you bubble ethane gas (CH3CH3, or EtH) through acids, bases, oxidizing agents, reducing agents—in fact almost any chemical you can think of—it will remain unchanged. Just about the only thing you can do with it is burn it. Yet ethanol (CH3CH2OH, or preferably EtOH— structure in the margin) not only burns, it reacts with acids, bases, and oxidizing agents.
The difference between ethanol and ethane is the functional group—the OH, or hydroxyl group. We know that these chemical properties (being able to react with acids, bases, and oxidizing agents) are properties of the hydroxyl group and not just of ethanol because other compounds containing OH groups (in other words, other alcohols) have similar properties, whatever their hydrocarbon frameworks.
Your understanding of functional groups will be the key to your understanding of organic chemistry. We shall therefore now go on to meet some of the most important functional groups. 



Alkanes contain no functional groups
The alkanes are the simplest class of organic molecules because they contain no functional groups. They are extremely unreactive and therefore rather boring as far as the organic chemist is concerned. However, their unreactivity can be a bonus, and alkanes such as pentane and hexane are often used as solvents, especially for the purification of organic compounds. Just about the only thing alkanes will do is burn—methane, propane, and butane are all used as domestic fuels, and petrol is a mixture of alkanes containing largely isooctane.

Alkanes contain no functional groups The alkanes are the simplest class of organic molecules because they contain no functional groups. They are extremely unreactive and therefore rather boring as far as the organic chemist is concerned. However, their unreactivity can be a bonus, and alkanes such as pentane and hexane are often used as solvents, especially for the purification of organic compounds. Just about the only thing alkanes will do is burn—methane, propane, and butane are all used as domestic fuels, and petrol is a mixture of alkanes containing largely isooctane.


Alkenes (sometimes called olefi ns) contain C=C double bonds
It may seem strange to classify a type of bond as a functional group, but you will see later that C=C double bonds impart reactivity to an organic molecule just as functional groups consisting of, say, oxygen or nitrogen atoms do. Some of the compounds produced by plants and used by perfumers are alkenes (see Chapter 1). For example, pinene has a smell evocative of pine forests, while limonene smells of citrus fruits.
You’ve already met the orange pigment
β-carotene. Eleven C=C double bonds make up most of its structure. Coloured organic compounds often contain chains or rings of C=C double bonds like this.

Alkenes (sometimes called olefi ns) contain C=C double bonds

Alkynes contain CC triple bonds
Just like C=C double bonds, CC triple bonds have a special type of reactivity associated with them, so it’s useful to call a CC triple bond a functional group. Alkynes are linear so we draw them with four carbon atoms in a straight line. Alkynes are not as widespread in nature as alkenes, but one fascinating class of compounds containing CC triple bonds is a group of antitumour agents discovered during the 1980s. Calicheamicin is a member of this group. The high reactivity of this combination of functional groups enables calicheamicin to attack DNA and prevent cancer cells from proliferating. For the first time we have drawn a molecule in three dimensions, with two bonds crossing one another—can you see the shape?

Alkynes contain C≡C triple bonds


Alcohols (R–OH) contain a hydroxyl (OH) group
We’ve already talked about the hydroxyl group in ethanol and other alcohols. Carbohydrates are peppered with hydroxyl groups; sucrose has eight of them, for example

sucrose

Molecules containing hydroxyl groups are often soluble in water, and living things often attach sugar groups, containing hydroxyl groups, to otherwise insoluble organic compounds to keep them in solution in the cell. Calicheamicin, a molecule we have just mentioned, contains a string of sugars for just this reason. The liver carries out its task of detoxifying unwanted organic compounds by repeatedly hydroxylating them until they are water soluble, and they are then excreted in the bile or urine.



Ethers (R1–O–R2) contain an alkoxy group (–OR)
The name ether refers to any compound that has two alkyl groups linked through an oxygen atom. ‘Ether’ is also used as an everyday name for diethyl ether, Et2O. You might compare this use of the word ‘ether’ with the common use of the word ‘alcohol’ to mean ethanol. Diethyl ether is a highly flammable solvent that boils at only 35°C. It used to be used as an anaesthetic. Tetrahydrofuran (THF) is another commonly used solvent and is a cyclic ether. Brevetoxin B (overleaf) is a fascinating naturally occurring compound that was synthesized in the laboratory in 1995. It is packed with ether functional groups in ring sizes from 6 to 8.

Ethers (R1–O–R2) contain an alkoxy group (–OR)



Amines (R–NH2) contain the amino (NH2) group
We met the amino group when we were discussing the amino acids: we mentioned that it was this group that gave these compounds their basic properties. Amines often have powerful fishy smells: the smell of putrescine is particularly foul. It is formed as meat decays. Many neurologically active compounds are also amines: amphetamine is a notorious stimulant.

Amines (R–NH2) contain the amino (NH2) group


Nitro compounds (R–NO2) contain the nitro group (NO2)
The nitro group (NO2) is sometimes incorrectly drawn with five bonds to nitrogen which as you will see in Chapter 4 is impossible. Make sure you draw it correctly when you need to draw it out in detail. If you write just NO2 you are all right!
Several nitro groups in one molecule can make it quite unstable and even explosive. Three nitro groups give the most famous explosive of all, trinitrotoluene (TNT), its kick. However, functional groups refuse to be stereotyped. Nitrazepam also contains a nitro group, but this compound is marketed as Mogadon
®, the sleeping pill.

Nitro compounds (R–NO2) contain the nitro group (NO2)

Nitro compounds (R–NO2) contain the nitro group (NO2)


Alkyl halides (fl uorides R–F, chlorides R–Cl, bromides R–Br, or iodides R–I) contain the fl uoro, chloro, bromo, or iodo groups
These four functional groups have similar properties, although alkyl iodides are the most reactive and alkyl fluorides the least. Polyvinyl chloride (PVC) is one of the most widely used polymers—it has a chloro group on every other carbon atom along a linear hydrocarbon framework. Methyl iodide (MeI), on the other hand, is a dangerous carcinogen since it reacts with DNA and can cause mutations in the genetic code. These compounds are also known as haloalkanes (fluoroalkanes, chloroalkanes, bromoalkanes, or iodoalkanes).

Alkyl halides (fl uorides R–F, chlorides R–Cl, bromides R–Br, or iodides R–I) contain the fl uoro, chloro, bromo, or iodo groups



Aldehydes (R–CHO) and ketones (R1–CO–R2) contain the carbonyl group C=O
Aldehydes can be formed by oxidizing alcohols—in fact the liver detoxifies ethanol in the bloodstream by oxidizing it first to acetaldehyde (ethanal, CH3CHO) (see p. 28). Acetaldehyde in the blood is the cause of hangovers. Aldehydes often have pleasant smells—2-methylundecanal is a key component of the fragrance of Chanel No. 5, and ‘raspberry ketone’ is the major component of the flavour and smell of raspberries.

Aldehydes (R–CHO) and ketones (R1–CO–R2) contain the carbonyl group C=O


Carboxylic acids (R–CO2H) contain the carboxyl group CO2H

As their name implies, compounds containing the carboxylic acid (CO2H) group can react
with bases, losing a proton to form carboxylate salts. Edible carboxylic acids have sharp flavours and several are found in fruits—citric, malic, and tartaric acids are found in lemons,
apples, and grapes, respectively.

Carboxylic acids (R–CO2H) contain the carboxyl group CO2H


Esters (R1–CO2R2) contain a carboxyl group with an extra alkyl group (CO2R)
Fats are esters; in fact they contain three ester groups. They are formed in the body by condensing glycerol, a compound with three hydroxyl groups, with three fatty acid molecules. Other, more volatile, esters have pleasant, fruity smells and flavours. These three are components of the flavours of bananas, rum, and apples:

Esters (R1–CO2R2) contain a carboxyl group with an extra alkyl group (CO2R)



Amides (R–CONH2, R1–CONHR2, or R1–CONR2R3)
Proteins are amides: they are formed when the carboxylic acid group of one amino acid condenses with the amino group of another to form an amide linkage (also known as a peptide bond). One protein molecule can contain hundreds of amide bonds. Aspartame, the artificial sweetener marketed as NutraSweet®, on the other hand, contains just two amino acids, aspartic acid and phenylalanine, joined through one amide bond. Paracetamol is also an amide.

Amides (R–CONH2, R1–CONHR2, or R1–CONR2R3)


Nitriles or cyanides (R–CN) contain the cyano group –CN
Nitrile groups can be introduced into molecules by reacting potassium cyanide with alkyl halides. The organic nitrile group has quite different properties from those associated with lethal inorganic cyanide: laetrile, for example, is extracted from apricot kernels, and was once developed as an anticancer drug.

Nitriles or cyanides (R–CN) contain the cyano group –C≡N



Acyl chlorides (acid chlorides, R–COCl)
Acyl chlorides are reactive compounds used to make esters and amides. They are derivatives of carboxylic acids with the –OH replaced by –Cl, and are too reactive to be found in nature.

Acyl chlorides (acid chlorides, R–COCl)


Acetals
Acetals are compounds with two single-bonded oxygen atoms attached to the same carbon atom. Many sugars are acetals, as is laetrile, which you have just met.

an acetal sucrose laetrile




Summary: Important functional groups and oxidation level

Zero bonds to heteroatoms: alkane oxidation level One bond to heteroatoms: alcohol oxidation level Two bonds to heteroatoms: aldehyde oxidation level Three bonds to heteroatoms: carboxylic acid oxidation level Four bonds to heteroatoms: carbon dioxide oxidation level R2 C R3 R1 R4 alkanes R OH R1 OR2 R NH2 R I R Br R Cl alcohols ethers amines alkyl halides R alkenes R1 C R2 R3O OR3 R H O R1 R2 O acetals ketones aldehydes R alkynes R OH O R1 OR2 O R NH2 O R C N R Cl O carboxylic acids amides esters nitriles acyl chlorides O C O C RO OR O C Cl Cl F F carbon dioxide carbonates tetrahalo compounds C RO OR O ureas






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