Q.What are aliphatic hydrocarbons? Compare the proportion of carbon and hydrogen atoms in ethane, ethene and ethyne. Which compound is most unsaturated with hydrogen?

CnH2n is the general molecular formula for the alkene homologous series.

Each class of hydrocarbon has a characteristic general formula reflecting its degree of unsaturation:

• Alkanes (saturated, only single bonds): CnH2n+2
• Alkenes (one C=C double bond): CnH2n
• Alkynes (one C≡C triple bond): CnH2n-2
• Arenes (aromatic, e.g. benzene series): CnH2n-6 …

C8H18 fits the alkane general formula CnH2n+2 exactly, so it is the aliphatic saturated hydrocarbon; the other formulas correspond to unsaturated or cyclic hydrocarbons.

An aliphatic hydrocarbon is an open-chain (non-aromatic) compound built only from carbon and hydrogen. It is saturated when every carbon-carbon bond is a single bond, which fixes its formula as CnH2n+2 (an alkane).

Check each option against CnH2n+2:

• C9H18: for n = 9, a saturated alkane would need 2(9)+2 = 20 hydrogens, but only 18 are present, so this formula is CnH2n — an alkene (or a saturated ring compound), not an open-chain saturated hydrocarbon.
• C8H14: for n = 8, an alkane would need 18 hydrogens; only 14 are present, so this is highly unsaturated (a diene, alkyne, or polycyclic structure). …

Alkynes (hydrocarbons with a C-C triple bond) follow the general formula CnH2n-2.

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond. Each triple bond accounts for 2 degrees of unsaturation relative to the saturated alkane CnH2n+2, so alkynes have 4 fewer hydrogens than the corresponding alkane: CnH2n+2 …

Ethyne is defined by its carbon-carbon triple bond, which only one of the four given structures actually has.

Ethyne (common name: acetylene) is the simplest member of the alkyne family, hydrocarbons characterised by at least one carbon-carbon triple bond (C≡C). Its structure is H-C≡C-H, molecular formula C2H2.

Checking the options:

• (a) HC≡CH: contains a C≡C triple bond — this is ethyne.
• (b) H2C=CH2: contains a C=C double bond — this is ethene (an alkene), not ethyne.
• (c) CH3-CH3: contains only a C-C single bond — this is ethane (an alkane).
• (d) CH3-CH2-CH3: contains only C-C single bonds, with 3 carbons — this is propane (an alkane). …