Compounds of carbon and hydrogen are called hydrocarbons. The simplest ones are called alkanes, in which all the C - C bonds are single bonds. They are said to be saturated because of this. The general formula of alkanes is CnH2n+2. A series of compounds with similar structure and chemical properties, in which members differ from one another by the possession of an additional CH2 group, is called a homologous series. The first ten members of the alkane series are:
|No. of carbon Atoms||Stem||Alkane||Formula|
The basis of naming organic compounds is that the name consists essentially of a Stem + Suffix. The stem gives the number of carbon atoms in the longest straight chain. The suffix gives the principal group, called functional group, or groups in the compound. Saturated hydrocarbons do not have a functional group and the suffix used for these compounds is -ane.
It is possible to have alkanes with straight or branched chains, e.g. it is possible for, C4H10, to have the atoms arranged in two different ways. These are called isomers.
Branched chains are named by regarding them as consisting of a straight chain with one or more side chains attached. There is a procedure to follow:
If there are more than one side groups, then separate numbers must appear in the name for each of them:
The alkanes are not the only hydrocarbons. There are also alkenes and alkynes. They are unsaturated hydrocarbons; they contain multiple bonds between carbon atoms. The simplest alkene is ethene, H2C=CH2. It is the first member of the homologous series of alkenes, which have the general formula CnH2n. Naming is similar to the alkanes but the suffix is now -ene instead of -ane. There is usually a number indicating the position of the double bond.
Alkynes contain one or more carbon-carbon triple bonds. The simplest is ethyne, HC≡CH. It is the first member of the homologous series of alkynes, which have the general formula, CnH2n-2.
Another set of hydrocarbons are the alicyclic hydrocarbons. These contain rings of carbon atoms.
These are related to benzene. Members of this group, the arenes, are benzene, C6H6; methyl benzene, C6H5CH3; naphthalene, C10H8.
The double bond in the alkenes is responsible for most of the chemical reactions of these compounds; in other words they contain the functional group . Similarly the alkynes contain the functional group .
The halogenoalkanes have the formula RX, where R is the alkyl group and X is a halogen. An alkyl group has the general formula CnH2n+1.
|Halogenoalkanes||Halogen||F, Cl, Br, I||Chloroethane C2H5Cl|
|Carboxylic acids||Carboxyl||-CO2H||Ethanoic acid CH3CO2H|
|Ethers||Ether||C - O - C||Methoxymethane CH3OCH3|
When a bond breaks, each of the bonded atoms takes one of the pair of electrons. Radicals are formed. These are atoms or groups of atoms with unpaired electrons.
Alkanes have two important uses: as fuels and as the foundation of the petrochemicals industry.
The reaction can continue until all the hydrogen atoms are substituted by chlorine atoms, yielding eventually, CCl4.
Much work has been done on how this reaction occurs, when methane is so unreactive towards other reagents. The reaction occurs rapidly in sunlight or above 300°C. It is an example of a photochemical reaction. The reaction proceeds by a number of steps involving radicals.
There are two isomers with the molecular formula C2H6O: namely ethanol, CH3CH2OH and methoxymethane, CH3OCH3.
Ethanol is an alcohol. All alcohols have the hydroxyl functional group, -OH, and exhibit similar chemical properties. Their general formula is R-OH.
Methoxymethane is an ether. All ethers contain the alkoxy functional group -OR and they shew similar chemical properties. Their general formula is R-O-R'.
Alcohols are derived from the alkanes by substituting an -OH group for a hydrogen atom.
For alcohols containing more than two carbon atoms, isomers are possible. To distinguish between them, it is necessary to label the position of the OH group. The hydrocarbon-chain is always numbered from the end which gives the lowest number for the position of the functional group. Some alcohols have more than one -OH group. These are called diols: for example CH2OHCH2OH is called ethane-1,2-diol.
Like water molecules, alcohols are polar due to the elctronegativity of the oxygen atom. Alcohols are therefore able to hydrogen bond to each other and to water molecules. This explains their solubility in water.
Ethers can be thought as being derived from water with both hydrogen atoms being replaced by alkyl groups. As there are no hydrogen atoms attached to the oxygen, so ethers cannot form hydrogen bonds between molecules. Ethers are only slightly soluble in water.
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