A compound made only of carbon and hydrogen is called a hydrocarbon. Hydrocarbons are the simplest carbon compounds and the parents of all others.
Saturated and unsaturated hydrocarbons
A saturated hydrocarbon contains only single bonds between carbon atoms; these are the alkanes, with the general formula CnH2n+2 (methane CH4, ethane C2H6, propane C3H8). An unsaturated hydrocarbon contains at least one double or triple bond. Those with a C=C double bond are alkenes (general formula CnH2n, e.g. ethene C2H4), and those with a C≡C triple bond are alkynes (general formula CnH2n-2, e.g. ethyne C2H2). Saturated hydrocarbons burn with a clean blue flame; unsaturated ones, having more carbon, burn with a yellow, sooty flame.
Chains, branches and rings
Carbon skeletons can be straight chains, branched chains or closed rings (cyclic compounds such as cyclohexane C6H12). The same atoms can be arranged in different ways.
Structural isomerism
Compounds that have the same molecular formula but different structures are called structural isomers. For example, C4H10 exists as two isomers: a straight chain (n-butane) and a branched chain (isobutane). Isomers have different physical properties even though they contain the same number of each kind of atom.
Functional groups
When a hydrogen atom of a hydrocarbon is replaced by another atom or group of atoms that gives the molecule characteristic properties, that atom/group is called a functional group. Important ones at this level are:
- Halo group (−Cl, −Br): haloalkanes, e.g. chloroethane C2H5Cl.
- Alcohol group −OH, e.g. ethanol C2H5OH.
- Aldehyde group −CHO, e.g. ethanal CH3CHO.
- Ketone group −CO−, e.g. propanone CH3COCH3.
- Carboxylic acid group −COOH, e.g. ethanoic acid CH3COOH.
Homologous series
A homologous series is a family of compounds with the same functional group, in which each member differs from the next by a −CH2 unit (a mass difference of 14 u). Members of a series share the same general formula and similar chemical properties, while their physical properties (melting point, boiling point) change gradually as the chain grows. For example, the alcohols methanol, ethanol and propanol form one homologous series.
Naming carbon compounds
The name has a root (meth-, eth-, prop-, but- for 1, 2, 3, 4 carbons), a suffix or prefix for the functional group, and an ending that shows saturation (−ane for single bonds, −ene for a double bond, −yne for a triple bond). For example, a 3-carbon compound with an −OH group is propan-1-ol, and a 2-carbon compound with a −COOH group is ethanoic acid.
Write the general formula of alkanes, alkenes and alkynes and give the molecular formula of the second member of each.
Solution- Alkanes: CnH2n+2; second member (n = 2) is ethane C2H6.
- Alkenes: CnH2n; second member (n = 3, since the smallest alkene has 2 C) — the second alkene is propene C3H6.
- Alkynes: CnH2n-2; the second alkyne is propyne C3H4.
Answer: Alkanes CnH2n+2 (ethane C2H6); alkenes CnH2n (propene C3H6); alkynes CnH2n-2 (propyne C3H4).
Distinguish between saturated and unsaturated hydrocarbons, including how their flames differ.
Solution- A saturated hydrocarbon (alkane) has only single C−C bonds; an unsaturated one (alkene or alkyne) has a double or triple bond.
- Saturated hydrocarbons burn with a clean blue flame.
- Unsaturated hydrocarbons have a higher proportion of carbon, so they burn with a yellow, sooty flame.
Answer: Saturated = only single bonds, blue flame; unsaturated = double/triple bond present, yellow sooty flame.
Draw (describe) the structural isomers of C4H10.
Solution- The molecular formula C4H10 can be arranged as a straight chain of four carbons: CH3−CH2−CH2−CH3 (n-butane).
- It can also be arranged as a branched chain with a central carbon carrying a CH3 branch (isobutane / 2-methylpropane).
- Both have the same molecular formula but different structures, so they are structural isomers.
Answer: Two isomers — n-butane (straight chain) and isobutane / 2-methylpropane (branched chain).
Identify the functional group and name the class of each: (i) C2H5OH, (ii) CH3COOH, (iii) CH3CHO.
Solution- C2H5OH has the −OH group, so it is an alcohol (ethanol).
- CH3COOH has the −COOH group, so it is a carboxylic acid (ethanoic acid).
- CH3CHO has the −CHO group, so it is an aldehyde (ethanal).
Answer: (i) −OH, alcohol; (ii) −COOH, carboxylic acid; (iii) −CHO, aldehyde.
Two consecutive members of a homologous series differ by a mass of 14 u. Explain why, and state the difference in their formulae.
Solution- Successive members of a homologous series differ by one −CH2 unit.
- The mass of a CH2 unit = 12 (C) + 2 × 1 (H) = 14 u.
- So each member is heavier than the one before it by 14 u and differs by CH2 in formula.
Answer: Consecutive members differ by a −CH2 group, which has a mass of 14 u.
Name the following: (i) a 3-carbon alkane, (ii) a 2-carbon alkene, (iii) a 3-carbon alcohol.
Solution- A 3-carbon saturated hydrocarbon uses the root prop- and the ending -ane: propane (C3H8).
- A 2-carbon hydrocarbon with a double bond uses root eth- and ending -ene: ethene (C2H4).
- A 3-carbon compound with an −OH group uses root prop- and suffix -ol: propan-1-ol (C3H7OH).
Answer: (i) propane, (ii) ethene, (iii) propan-1-ol.