E1. Write the structural formula of the heptane and 2,2,4-trimethylpentane molecules.
EA1.
E2. What is the name of the straight-chain hydrocarbon that has same molecular formula as 2,2,4-trimethylpentane?
EA2. C8H18 octane
Q1. Give the meaning of the following terms: a homologous series b structural isomers c structural formula d semistructural formula e saturated f unsaturated
A1. a Homologous series: A series of organic compounds in which each members
differs by a CH2– group from the previous members. Members of a homologous series have similar chemical properties.
b Structural isomers: Molecules with the same formula but different molecular structures.
c Structural formula: A formula that represents the three-dimensional arrangement of atoms in a molecules.
d Semi-structural formula: A formula that shows the sequence of atoms in a molecules without indicating the three dimensional arrangement of the atoms within the molecule.
e Saturated: Carbon compounds that only contain single bonds between the carbon atoms.
g Unsaturated: Carbon compound that contain at least one double or triple bond between adjacent carbon atoms.
Worked solutions to student book questions Chapter 9 Compounds of carbon
Q4. Explain why there is only one compound corresponding to the formula C3H8 while there are over 70 compounds corresponding to the molecular formula C10H22.
A4. The number of possible ways of arranging carbon atoms within a molecule increases as the number of carbon atoms increase.
Q5. Describe and explain the difference in bonding and structure between alkanes and alkenes.
A5. In alkanes, there is a single bond between all the carbon atoms. There is a tetrahedral arrangement of the bonds formed by each carbon atom. Alkenes contain at least one double bond between adjacent carbon atoms. The bonds formed around the double-bonded carbon atoms lie in the same plane.
Q6.
Why does the alkene homologous series begin with ethene, C2H4, while the alkanes start with methane, CH4?
A6.
Alkenes have a double bond between two adjacent carbon atoms. Hence the smallest alkene is ethene, CH2=CH2.
Worked solutions to student book questions Chapter 9 Compounds of carbon
Q10. Why don’t we use the names 1-chloroethane or 3-propanol?
A10. Since 1-chloroethane and 2-chloroethane are identical, the molecule is simply named chloroethane. In the case of 3-propanol, this molecule is identical to 1-propanol and it is called 1-propanol by convention. (The numbers used are always the smaller values.)
Q11. The semi-structural formulas of some organic compounds is given below. For each compound: i identify the homologous series to which it belongs ii give its systematic name. a CH3(CH2)5CH2Cl b CH3(CH2)2CHCl(CH2)2CH3 c CH3CHOH(CH2)4CH3 d CH3(CH2)4COOH e CH3(CH2)2CH(NH2)CH3 f (CH3)2CHCH2CH3 g (CH3)2C=CH2
Q19. Table 9.7 gives the molecular masses and boiling points of butane, propan-1-ol and chloroethane. Explain why the boiling points differ even though the compounds have similar molecular masses. Table 9.7
Compound Molecular mass Boiling point (°C) Butane 58 –0.5 Propan-1-ol 60 97.0 Chloroethane 65 12.5
A19. The differences in boiling points is indicative of the strength of bonds between molecules. There a weak dispersion forces between butane molecules. In chloroethane, the bond between chlorine and carbon is polar and there are dipole-dipole bonds between chloroethane molecules resulting in it having a higher boiling point than butane. Hydrogen bonds exist between the propan-1-ol molecules due to the electronegative nature of the oxygen atom. These bonds are stronger than the bonds between chloroalkane molecules and the bonds between butane molecules.
Q20. Explain why alkanols such as methanol and ethanol are soluble in water in all proportions whereas the alkanols higher in the homologous series such as 1-octanol are insoluble.
A20. Hydrogen bonds form between the polar hydroxy function group in alkanol in alkanol and water molecules. The alkyl group is non polar and does not interact with water molecules. As the size of the alkyl group increases, the solubility of the alkanol in water decreases.
Q21. Write an equation for the ionisation of methanoic acid in water.
A21.
HCOOH(l) + H2O(l) → HCOO–(aq) + H3O+(aq)
Worked solutions to student book questions Chapter 9 Compounds of carbon
Q22. With the aid of a diagram, explain why methylamine is soluble in water.
A22. Bonds around the N atom in methylamine are polar due to the highly electronegative nature of the N atom. Methylamine is soluble in water because hydrogen bonds form between methylamine and water molecules.
Q23. Why would ethanamine be more soluble in water than butanamine?
A23. The highly polar amino functional group can form hydrogen bonds with water molecules but as the non-polar hydrocarbon chain grows longer, as in butanamine, the solubility is reduced.
Q24. Write equations for the the reaction between ethanoic acid solution and: a sodium hydroxide solution b solid sodium carbonate c magnesium
A24.
a CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l) b 2CH3COOH(aq) + Na2CO3(s) → 2CH3COONa(aq) + CO2(g) +H2O(l) c 2CH3COOH(aq) + Mg(s) → (CH3COO)2Mg + H2(g)
Worked solutions to student book questions Chapter 9 Compounds of carbon
Q25. Prepare a poster that summarises the rules for the systematic naming of carbon compounds.
A25. The poster should include reference to the following. Alkanes:
• The first part of the name indicates the number of carbon atoms. • The last part ends in -ane. • The stem of the name of branched alkanes is derived from longest alkane
hydrocarbon chain. The name and number indicating the position of any akyl side chains is then added.
Alkenes: • The names of alkenes end in -ene. • The stem of the systematic name of alkenes is based on the longest carbon
chain that contains the double bond. • The position of the double bond is indicated by the number of the first carbon
atom involved in the double bond. By convention, numbering starts from the end nearest the double bond. The rules for naming of any side chains are similar to those for the alkanes.
Functional groups: • Numbers are used to indicate the position of the functional group attached to
the carbon chain • Chloroalkanes: the name starts with chloro- followed by the name of the
alkane from which it is derived. • Alkanols are named by dropping the ‘e’ at the end of the hydrocarbon name
and replacing it with ‘ol’. • Carboxylic acid: The name of carboxylic acids is determined from the total
number of carbon atoms in the molecule, and -oic acid is added at the end of the name.
• Amines are named by adding -amine to the alkane stem.
Q26.
Prepare notes for a PowerPoint presentation that outlines the reasons why carbon is able to form so many different compounds.
A26. The notes might include.
• Carbon forms stable bonds with other carbon atoms, and can form large chains as well as cyclic structures.
• Carbon forms single, double or triple bonds with other carbon atoms. • Carbon is able to bond with the atoms of other elements, H, O, N, P, Cl, and
with groups of atoms (functional groups). • There are different organic compounds that have the same molecular formula
