Exam practice

In ethane (CH₃–CH₃), the two carbon atoms are joined by a single covalent bond, and each carbon’s other three bonds are already taken by hydrogen atoms. The carbons hold the most hydrogen they possibly can, so no more atoms can be added: the molecule is “saturated”.

A glycerol molecule, C₃H₅(OH)₃, contains three hydroxyl (–OH) groups, one on each carbon atom. An alcohol with three –OH groups is called trihydric.

Hydrocarbons are organic compounds made of only carbon and hydrogen atoms. Examples: methane (CH₄), ethane (C₂H₆), ethene (C₂H₄), ethyne (C₂H₂). (Also propane, butane.)

A saturated hydrocarbon is a hydrocarbon in which the carbon atoms are joined only by single covalent bonds, so each carbon holds as many hydrogen atoms as possible. They are the alkanes, CₙH₂ₙ₊₂, and are stable and not very reactive. Examples: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.

Methane, CH₄, is the simplest hydrocarbon and the first alkane: one carbon atom joined to four hydrogen atoms by single bonds. It is formed when plant matter rots without air, so it is found in marshes (“marsh gas”), in biogas, sewage gas and natural gas. It is a colorless, odorless, tasteless gas, insoluble in water, that burns with a blue flame.
Uses:

1. as a fuel: biogas for cooking, natural gas in industry;
2. to make carbon black for printing ink, shoe polish and paint.

(Also: to make chloroform and methanol, and hydrogen.)

Alcohols are organic compounds formed when one or more hydrogen atoms of an alkane are replaced by hydroxyl (–OH) groups. Their functional group is –OH.

• Monohydric (one –OH): methanol, CH₃OH; ethanol, C₂H₅OH.
• Dihydric (two –OH): glycol (ethane-1,2-diol), C₂H₄(OH)₂.
• Trihydric (three –OH): glycerol (propane-1,2,3-triol), C₃H₅(OH)₃.

Structural formulas: as in Figure 18.6. Ethanol, for example, is H₃C–CH₂–OH drawn out with every bond: two carbons in a row, three H on the first, two H on the second, and the –OH at the end.

Structural formulas: ethene, propene and ethyne as in Figure 18.4; ethanol and glycerol as in Figure 18.6. Check each drawing: every C has four bonds, every H one, every O two.

• i.Methyl alcohol (methanol).
• ii.Ethyl alcohol (ethanol).
• iii.Ethyl alcohol (ethanol).
• iv.Ethyl alcohol (ethanol).

Ethylene (ethene), C₂H₄: two carbon atoms joined by a double bond, each carrying two hydrogen atoms (as in Figure 18.4): H₂C=CH₂.
Each hydrogen is joined to carbon by a single covalent bond.
The two carbons share two pairs of electrons. The second pair isn’t held as tightly as the first: it lies above and below the line between the two carbons, where other molecules can reach it. So one of the two bonds breaks easily, and other atoms (such as hydrogen or bromine) can add on. That is what is meant when the double bond is called “weak”: the double bond as a whole is stronger than a single bond, but it is less than twice as strong, and its second bond is the reactive part that makes alkenes unsaturated and reactive.

• i.Ethane, C₂H₆.
• ii.Yes. The two carbon atoms are joined by a single covalent bond, and each carbon’s remaining bonds are all taken by hydrogen; it fits CₙH₂ₙ₊₂ (n = 2, H = 6).
• iii.Ethanol (ethyl alcohol), C₂H₅OH.

1. As a fuel, in biogas for cooking and natural gas for industry.
2. To make carbon black, used in printing ink, shoe polish and paint.
3. To make chloroform, carbon tetrachloride, methanol and formaldehyde. (Also: to make hydrogen gas.)

Ethane: as a fuel that gives heat when burned; to make other carbon compounds such as ethyl chloride and nitroethane (and, above all, ethene for plastics).

Propane: as a fuel (in LPG); to make other carbon compounds; as a coolant in the petroleum industry.

• i.Saturated hydrocarbon: a hydrocarbon with only single covalent bonds between its carbon atoms.
• ii.Unsaturated hydrocarbon: a hydrocarbon with at least one double or triple covalent bond between carbon atoms.
• iii.Alkane: a saturated hydrocarbon, with general formula CₙH₂ₙ₊₂ (e.g. methane).
• iv.Alkene: an unsaturated hydrocarbon with one carbon–carbon double bond, general formula CₙH₂ₙ (e.g. ethene).
• v.Alkyne: an unsaturated hydrocarbon with one carbon–carbon triple bond, general formula CₙH₂ₙ₋₂ (e.g. ethyne).
• vi.Functional group: an atom or group of atoms that decides the structure and chemical properties of a family of organic compounds (e.g. –OH in alcohols).
• vii.Homologous series: a series of organic compounds with the same general formula and functional group, in which neighboring members differ by a CH₂ group (14 in molecular mass), e.g. methane, ethane, propane.

Glycerol C₃H₅(OH)₃ (or C₃H₈O₃); methane CH₄; ethanol C₂H₅OH; butane C₄H₁₀; propane C₃H₈; acetylene C₂H₂; ethene C₂H₄; ethane C₂H₆; propyne C₃H₄; methyl alcohol CH₃OH.

Ethanol (ethyl alcohol), C₂H₅OH, usually as methylated spirit (ethanol with some methanol added). Structural formula: as in Figure 18.6: H₃C–CH₂–OH with all the bonds drawn.

Alkanes (saturated hydrocarbons, CₙH₂ₙ₊₂) are called paraffins. The name comes from Latin parum affinis, “little affinity”: with only single bonds, alkanes react with very few substances. (Paraffin wax and the paraffin oil sold as kerosene are mixtures of alkanes.)

Glycerol (glycerine), C₃H₅(OH)₃. IUPAC name: propane-1,2,3-triol. Structural formula: three carbons in a row, with one –OH on each, as in Figure 18.6.
Uses:

1. in creams and lotions, to keep the skin moist;
2. in good-quality soaps, cosmetics and shaving creams;
3. as a sweetener in foods, and to keep dried fruit and tobacco moist.

Glycerol (glycerine). It attracts and holds water, so it keeps the skin moist and soft. IUPAC name: propane-1,2,3-triol; formula C₃H₅(OH)₃; structural formula as in Figure 18.6.