Alkanes – answers to problems
1
(c) and (d) are the same. [If two molecules can only be interconverted by
breaking a bond and then reforming it somewhere else in the molecule, they
are structural isomers. If two structures just differ in the way the atoms are
shown on the page, but not in the arrangement of the bonds, the molecules
are the same.]
2
In this style of representing a molecule a carbon atom is at
every vertex, so the molecule shown contains six carbon atoms. The longest
carbon chain contains four carbons, so this is a derivative of butane. Two
methyl groups are attached to the second carbon atom in the chain, so the
molecule is 2,2-dimethylbutane.
3
4
(a)
Propagation steps are those in which the unpaired electron is not lost.
See the text, or the lecture slides, for an example.
(b)
Various means can be used. Light in the violet or ultraviolet region of
the spectrum is of high enough energy to break the bond in both Cl2
and I2, so this provides a convenient way to start the reaction. Heating
will work, but is unpredictable (you can fire in a flash of light to get the
reaction going at exactly the time you want, but if you are heating the
mixture instead you have no idea just when reaction will begin). The
use of a free-radical initiator, such as a peroxide, is also often
convenient.
(c)
Chlorine can react with methane to give chloromethane, but the
chloromethane is just as reactive as the methane was, so if there is
some spare chlorine present, further chlorine atoms can be added to
give dichloro-, trichloro- or tetrachloro-methane. If there is a large
excess of chlorine the tetrachloro- product will be formed almost
exclusively, but if the amounts of methane and chlorine are similar,
some mono-, di-, tri- and tetra-substituted methane will be formed,
mixed with some unreacted methane.
(d)
In order to form carbon tetrachloride as the major product, a high ratio
of chlorine to chloromethane would be required.
(a)
C5H12
(b)
C5H12
(b)
C5H10
1
5
6
(a)
A compound that contains only hydrogen and carbon.
(b)
Yes.
(c)
hexane: molecular formula = C6H14, empirical formula = C3H7. The
molecular formula shows the actual number of each type of atom in
one molecule of the compound. The empirical formula shows the lowest
whole-number ratio of the atoms. Often the two are the same, but if
all the coefficients in the molecular formula share an integer factor
greater than one the two will be different.
(a)
Butane and 2-methylpropane are structural isomers. The London
dispersion forces in butane are greater than those in 2-methylpropane
because butane molecules are longer and thinner, so when two butane
molecules are side by side, a greater number of atoms can interact via the
dispersion forces than in a couple of side-by-side 2-methylpropane
molecules, which is more globular. The energy associated with this stabilising
interaction needs to be overcome when butane boils, since when boiling
occurs each molecule separates from its neighbours, overcoming the force
that tends to hold them to each other. The higher boiling point of butane
reflects this extra energy.
(b) Higher. Dispersion forces increase with molecular weight since
molecules of higher molecular weight generally have more electrons than
lighter molecules, and usually the dispersion force increases as the number
of electrons increases.
7
In the eclipsed form the two OH groups are quite close to each other and can
form a hydrogen bond, which stabilizes this conformer. If the two ends of
the molecule rotated by, say, 180o, the two –OH groups would not be close
enough to form a hydrogen bond and the molecule would be less stable. Of
course the use in toothpaste and anti-freeze has nothing to do with the
question.
8
Since all the carbon and hydrogen within the alkane becomes CO2 and H2O
in the product, we can use the mass of each product to calculate the molar
ratios of C : H.
Number of moles CO2 = 4.800 g/42 g mol–1 = 0.11 moles. Since there is
one mole of carbon per mole of CO2, this is the number of moles of C in the
sample of alkane that was burnt.
Number of moles H2O = 2.475 g/18 g mol–1 = 0.1375 moles. Since there are
two moles of H (atoms) per mole of water, the number of moles of H atoms
in the sample of alkane is twice this figure, = 0.275 moles.
Ratio C : H = 0.11 : 0.275 = 1 : 2.5 or 2 : 5.
The general molecular formula for an aliphatic alkane is C nH2n+2, trial and
error will show that the only possible molecular formula is C4H10.
9
Ask!
10
(a)
Greater than 2:1 (hexane:chlorine) to reduce the chance that a
molecule that has already been chlorinated will meet more chlorine
and react a second time.
(b)
The carbons (and hydrogens) in cyclohexane are all chemically
equivalent, so monochlorination anywhere on the ring leads to the
same product. By contrast, in hexane there are three types of carbons
(positions 1, 2 and 3 in the chain). The secondary carbons are the
more reactive, so two different products are formed in quantity. Some
reaction at primary carbons will also occur. Reaction of chlorine with
either the cyclic or the straight chain compound will also produce some
molecules containing more than one chlorine atom, though this can be
reduced by having a significant excess of the hydrocarbon.