Definition
Organic chemistry is the chemistry of carbon compounds. A few carbon compounds — carbonates, cyanides, carbides, cyanates and the oxides of carbon — are counted as inorganic.
Catenation: carbon atoms form strong covalent bonds with one another, giving long chains (straight or branched) and rings. This is why carbon alone forms far more compounds than all other elements together.
Remember
Carbon sits near the middle of the periodic table, so it neither loses nor gains four electrons to form ionic bonds — it shares its four outer electrons and forms four covalent bonds. Organic compounds are therefore covalent compounds: generally low melting and boiling points, and usually insoluble in water.
Definition
Hydrocarbons are organic compounds made of carbon and hydrogen only (natural gas, LPG, petrol, diesel, kerosene). On the basis of structure they are divided into alkanes, alkenes, alkynes and aromatic hydrocarbons.
Alkanes contain only C–C and C–H single bonds, so they are called saturated hydrocarbons. General formula: . Methane () is the first member.
Rule
IUPAC name = prefix + root + suffix. Root: number of carbon atoms in the longest continuous chain (meth-1, eth-2, prop-3, but-4, pent-5, hex-6, hept-7, oct-8, non-9, dec-10). Suffix: the family (-ane for alkanes). Prefix: the groups attached to the chain.
Number the longest chain from the end nearer to the branch.
Example 1 — naming a branched alkane
Give the IUPAC name of .
- The longest continuous chain has carbon atoms, and the compound is an alkane: butane.
- One methyl () group is attached as a branch.
- Numbering from the end nearer to the branch puts the methyl group on carbon .
Answer: 2-Methylbutane
Rule
Cracking of higher hydrocarbons: naphtha heated to about with a catalyst (zeolite) breaks into smaller hydrocarbons — a mixture of alkanes and alkenes with to carbon atoms.
Reduction (hydrogenation) of alkenes and alkynes with hydrogen over nickel at : and . Methane cannot be made this way.
Reduction of alkyl halides with Zn/HCl: zinc and hydrochloric acid give atomic hydrogen, which reduces the alkyl halide: .
Remember
Alkanes are also called paraffins ("little affinity") because they hardly react: the electronegativities of carbon () and hydrogen () are close, so C–H and C–C bonds are almost non-polar and acids, bases and oxidising agents find no site to attack.
Rule
Halogenation (a photochemical substitution reaction, in sunlight): a hydrogen atom of the alkane is replaced by a halogen atom. , and the reaction goes on to give , (chloroform) and (carbon tetrachloride).
Combustion: alkanes burn in air to give carbon dioxide and water with a large amount of heat. .
Example 2 — oxygen needed to burn propane
How many moles of oxygen are needed to burn one mole of propane completely?
- Write the balanced equation: .
- The mole ratio of propane to oxygen is .
Answer: moles of
Quick Recap
- ●Organic chemistry: carbon compounds; catenation gives chains and rings
- ●Alkanes: saturated hydrocarbons, single bonds only,
- ●IUPAC name: prefix (branches) + root (carbons in longest chain) + suffix (-ane)
- ●Preparation: cracking of naphtha; hydrogenation of alkenes/alkynes (Ni, ); reduction of alkyl halides (Zn/HCl)
- ●Reactions: halogenation (substitution, in sunlight) and combustion ( + + heat)
