ORGANIC CHEMISTRY (HYDROCARBON)

Organic chemistry is the study of compounds of carbon. Nearly

all organic compounds also contain hydrogen; most also contain oxygen, nitrogen, or other elements . 

Hydrocarbons are the simplest organic compounds . Containing only carbon and hydrogen, they can be straight-chain, branched chain, or cyclic molecules. Carbon tends to form four bonds in a tetrahedral geometry. Hydrocarbon derivatives are formed when there is a substitution of a functional group at one or more of these positions.

 

The majority of hydrocarbons found naturally occur in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen which, when bonded, can catenate to form seemingly limitless chains. 

Hydrocarbon frameworks

Chains

The simplest class of hydrocarbon frameworks contains just chains of atoms.

Names for carbon chains

 

Functional groups

1.      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 purification of organic compounds.

2.      Alkenes (sometimes called olefins) 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

3.      Alkynes contain CºC triple bonds

Just like C=C double bonds, CºC triple bonds have a

special type of reactivity associated with them, so it’s

useful to call a CºC triple bond a functional group.

4.       Alcohols (R–OH) contain a hydroxyl (OH) group

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.

5.       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.

6.       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.

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

Several nitro groups in one molecule can make it quite unstable and even explosive. Three nitro

groups give the most famous explosive of all, TNT (trinitrotoluene), its kick.

8.      Alkyl halides (fluorides R–F, chlorides R–Cl, bromides R–Br, or iodides R–I)

contain the fluoro, chloro, bromo, or iodo groups

These three functional groups have similar properties—though alkyl iodides are the most reactive

and alkyl fluorides the least. PVC (polyvinyl chloride) 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

9.      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). Acetaldehyde in the blood is the cause of hangovers.

1       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.

1      Esters (R1–CO2R2) contain a carboxyl group with an extra alkyl group (CO2R)
more volatile esters, have pleasant, fruity

smells and flavours. These three are components of

the flavours of bananas, rum, and apples:

1     Amides (R–CONH2, R1–CONHR2, or R1CONR2R3)

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).

1      Nitriles or cyanides (R–CN) contain the cyano group –CºN

Nitrile groups can be introduced into molecules by reacting potassium cyanide with alkyl halides.

The organic nitrile group has quite different properties associated with lethal inorganic cyanide:

Laetrile

1   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.

1   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.

Covalent Bonding in Organic Compounds

A carbon can

have four single bonds, two double bonds, a double and two single bonds, or

a triple and a single bond; all total four bonds. These bonds can be

represented by electron dot or line bond formulas.

Electron Configuration of Carbon

Bonding in carbon involves the promotion of a 2s electron to an empty 2p

orbital thus creating four unpaired electrons, one in the 2s and one in each of

the three 2p orbitals. This allows carbon to be tetravalent.

Shapes of Organic Molecules

The shapes of organic molecules are predicted using the following

principle: atoms and non-bonding electron pairs attached to a common

central atom are arranged as far apart in space as possible. If there are four

surrounding groups, the shape is tetrahedral; with three, the groups protrude

to the corners of a triangle (trigonal); and with two, the region is linear.

Bonding in Organic Compounds – A Summary

A carbon with four bonded groups is tetrahedral, sp3-hybridized, and has

109.5O bond angles. A carbon with three is trigonal, sp2-hybridized, and has

120O bond angles. A carbon with two bonded groups is linear, sp-hybridized,

and has 180O bond angles.

A single bond is a sigma bond; a double bond is composed of one sigma

bond and one pi-bond; a triple bond is one sigma and two pi-bonds.

Triple bonds are stronger than double bonds and double bonds are

stronger than single bonds. The opposite order describes relative bond

lengths.