A Data Booklet may be used.
Source: Learners' Alley, Alkanes slides
Source: Learners' Alley, Alkanes slides
CommonFind the statistical ratios of
Source: Learners' Alley, Alkanes slides
Source: Learners' Alley, Alkanes slides
Source: Learners' Alley, Alkanes slides
Source: Learners' Alley, Alkanes slides
Source: Learners' Alley, Alkanes slides
CommonExcess pentane was reacted with bromine in the presence of UV light. Assuming that only mono-bromination takes place and the reaction occurs at the same rate at all carbon atoms, what is the ratio of the products formed?
| 1-bromopentane | 2-bromopentane | 3-bromopentane | |
|---|---|---|---|
| A | 1 | 3 | 2 |
| B | 3 | 1 | 2 |
| C | 1 | 2 | 3 |
| D | 3 | 2 | 1 |
Source: VJC 2012 Prelim P1 Q18, via EJC Alkanes Topical Revision, Q4
CommonAlkanes differ in their boiling points. Four alkanes are given below.
Which of the following shows the correct sequence of the boiling points of the four alkanes given?
Source: CJC 2013 Prelim P1 Q15, via EJC Alkanes Topical Revision, Q5
Name the following compounds.
Source: RJC H2 Chemistry Tutorial 11 (Alkanes), Q1(a) and (b)
Source: RJC H2 Chemistry Tutorial 11 (Alkanes), Q2
Source: RJC H2 Chemistry Tutorial 11 (Alkanes), Q4
ChallengeBromination of 3-methylpentane produces a mixture of four different monobromoalkanes. One of them is compound X.
Source: EJC Alkanes Topical Revision, Section B Q8(b)(i) to (iii)
2,3-dimethylbutane[1]
x = 30 ÷ 10 = 3[1]
x + y4 = 50 ÷ 10 = 5, so y = 8[1]
C3H8, propane[1]
Pentane has a larger surface area of contact, so stronger instantaneous dipole–induced dipole interactions[1]
more energy is needed to overcome them.[1]
Br• + CH4 → •CH3 + HBr[1]
•CH3 + Br2 → CH3Br + Br•[1]
Limited halogen (excess alkane).[1]
Energy released forming permanent dipole–induced dipole interactions is insufficient to overcome the instantaneous dipole–induced dipole interactions between alkane molecules[1]
and the hydrogen bonds between water molecules.[1]
D3 : 2 : 1[1]
By probability, there are 6 possible (1°) hydrogen atoms for substitution that lead to 1-bromopentane, 4 possible (2°) hydrogen atoms that lead to 2-bromopentane, and 2 possible (2°) hydrogen atoms that lead to 3-bromopentane.
B1 < 3 < 4 < 2[1]
The intermolecular forces between alkane molecules are instantaneous dipole–induced dipole interactions. The weaker or less extensive the intermolecular forces between the alkane molecules, the lower the boiling point.
Alkane 1 has the smallest electron cloud (34 electrons, against 42 for the rest), so the weakest instantaneous dipole–induced dipole interactions. Alkane 1 has the lowest boiling point.
Among alkanes 2 to 4, alkane 2 is the only straight-chained molecule and so has the most extensive instantaneous dipole–induced dipole interactions. Alkane 3 is more branched than alkane 4, so it has the least extensive instantaneous dipole–induced dipole interactions of the three. The boiling points increase as 3 < 4 < 2.
Boiling point: (e) < (d) < (c) < (a) < (b), i.e. 2-methylpentane (60 °C) < hexane (69 °C) < 3,3-dimethylpentane (86 °C) < 2-methylhexane (91 °C) < heptane (98 °C)[1]
(c), (a) and (b) have higher boiling points than (e) and (d) because of their larger molecular size (7 C against 6 C). Their electron clouds are larger and more easily polarised, so the intermolecular instantaneous dipole–induced dipole interactions (instantaneous dipole–induced dipole interactions) are more extensive. Since boiling involves overcoming intermolecular forces, the boiling points of (c), (a) and (b) are higher.[1]
For (c), (a) and (b) (same number of C atoms), the degree of branching decreases from (c) to (a) to (b), so there is a greater surface area of contact between molecules, more extensive instantaneous dipole–induced dipole interactions and hence a higher boiling point from (c) to (a) to (b).[1]
Likewise, the degree of branching decreases from (e) to (d), so there is a greater surface area of contact between molecules of (d), more extensive instantaneous dipole–induced dipole interactions and hence a higher boiling point for (d) than (e).[1]
CxHy + (x + y4)O2 → xCO2 + y2H2O(l). Volumes / cm3: CxHy 30, O2 (340 − 100) = 240, CO2 (250 − 100) = 150. Mole ratio 1 : 8 : 5
1 mol of CxHy produces x mol of CO2, hence x = 5. Also, (x + y4) = 8, so y = 12.[1]
The alkane is C5H12. Mr = 72.0[1]
There are 3 isomers: CH3CH2CH2CH2CH3 pentane, CH3CH(CH3)CH2CH3 2-methylbutane, C(CH3)4 2,2-dimethylpropane[1]
Name of mechanism: free radical substitution.
Initiation: [1]
Propagation: [1]
Termination: [1]
| structure | relative proportion |
|---|---|
| 6 | |
| 4 | |
| 1 | |
| 3 |
Three structures with their proportions[3]