I've taken a couple of days to think about this question in the context of what is reasonable at year 12 HSC and even IB level. In general, I think that an exam asking about it would be problematic, though there is no reason for
@hs17 not to raise it.
To explore it a little, let's look at some actual data. There is a threshhold question here of whether a claim of a general trend BP(alkene) < BP(alkane) < BP(alkyne) is even true. I think that there are some caveats missing, such as:
- that species of approximately equal molar mass are intended
- that comparisons are including only one functional group - a carbon-to-carbon double bond in the alkene and a carbon-to-carbon triple bond in the alkyne - so there are no complications from other functionalities
- that the carbon skeletons are directly comparable
Without these, the general rule suggested might be used to say that cyclohexene (a liquid at RT) should have a lower boiling point than methane, despite the latter being a gas. Similarly, I am sure that glycerol (1,2,3-propanetriol) has a higher boiling point than propyne.
After taking this into account, we have a claim that the trend is evident when comparing compounds of similar structure and the same number of carbons. Well, this we can check:
} \quad &< \quad \text{ethane (BP:} -88.5\ ^{\circ}\text{C)} \quad &&< \quad \text{ethyne (BP:} -84.7\ ^{\circ}\text{C)} \\ \text{propene (BP:} -48.0\ ^{\circ}\text{C)} \quad &< \quad \text{propane (BP:} -43.8\ ^{\circ}\text{C)} \quad &&< \quad \text{propyne (BP:} -23.2\ ^{\circ}\text{C)} \\ \text{1-butene (BP:} -6.2\ ^{\circ}\text{C)} \quad &< \quad \text{butane (BP:} -0.5\ ^{\circ}\text{C)} \quad &&< \quad \text{1-butyne (BP:} +8.0\ ^{\circ}\text{C)} \\ \text{1-pentene (BP:} +29.9\ ^{\circ}\text{C)} \quad &< \quad \text{pentane (BP:} +36.0\ ^{\circ}\text{C)} \quad &&< \quad \text{1-pentyne (BP:} +40.1\ ^{\circ}\text{C)} \\ \text{1-hexene (BP:} +63.4\ ^{\circ}\text{C)} \quad &< \quad \text{hexane (BP:} +68.7\ ^{\circ}\text{C)} \quad &&< \quad \text{1-hexyne (BP:} +71 \text{ to } +72\ ^{\circ}\text{C)} \end{align*})
The general rule appears accurate, with the alkene generally having a BP a little below that of the corresponding alkane, and the corresponding alkyne having a BP a few degrees higher.
The point that
@Qeru was making is seen, too. Looking along the trends of alkenes or alkanes or alkynes, BP increases with chain length due to the increase in number and overall strength of dispersion force interactions seen with greater molar mass. Within the groups with the same number of carbon atoms, this effect predicts that the alkane will have a higher BP than the alkene (which it does), but predicts that the alkyne should be lower still, which it is not - and this is what has hs17 puzzled, I assume.
The BP that catches my eye is that of propyne, which is about 20 °C above propane, a far larger gap than is seen in the other cases. It is also worth considering which is the alkene / alkyne that corresponds to a given alkane. Taking the 4 carbon case, for example, we find:
} \quad < \quad \text{butane (BP:}\ -0.5\ ^{\circ}\text{C)} \quad < \\ \text{(} \textit{E} \text{)-2-butene (BP:}\ +3.7\ ^{\circ}\text{C)} \quad = \quad \text{(} \textit{Z} \text{)-2-butene (BP:}\ +3.7\ ^{\circ}\text{C)} \quad < \\ \text{1-butyne (BP:}\ +8.0\ ^{\circ}\text{C)} \quad < \quad \text{2-butyne (BP:}\ +26.9\ ^{\circ}\text{C)})
The compounds with internal multiple bonds have BPs higher than their isomers with the multiple bond at carbon 1, and the internal alkenes both have a BP higher than that of the alkane, inconsistent with the general rule. We also see the much higher BP gap from butane to 2-butyne, analogous to the gap from propane to propyne.
Checking the 5 carbon cases, we see similar results:
} \quad < \quad \text{pentane (BP:}\ +36.0\ ^{\circ}\text{C)} \quad < \\ \text{(} \textit{E} \text{)-2-pentene (BP:}\ +36.7\ ^{\circ}\text{C)} \quad < \quad \text{(} \textit{Z} \text{)-2-pentene (BP:}\ +36.9\ ^{\circ}\text{C)} \quad < \\ \text{1-pentyne (BP:}\ +40.1\ ^{\circ}\text{C)} \quad < \quad \text{2-pentyne (BP:}\ +56.1\ ^{\circ}\text{C)})
Pentane's BP is above that of 1-pentene but below the two 2-pentene isomers, which are a little below 1-pentyne and well below 2-pentyne.
In the 6 carbon cases, 1-hexene has the lowest boiling point at 63.4 °C, the 3-hexenes have BP below hexane, as does (
E)-2-hexene, but (
Z)-2-hexene is just above hexane (these five compounds all have BPs between 66 and 70 °C), and then come the hexynes: 1-hexyne at 71 to 72 °C, 3-hexyne at 81 °C, and finally 2-hexyne at 84.5 °C.
So, looking at the threshhold question, I am not convinced that this general rule is true, unless stated as something like
BP(1-alkene CnH2n) < BP(alkane CnH2n+2) BP(1-alkyne CnH2n-2)
because the internal alkenes do appear to be comparable to their alkane analogue. The alkynes are systematically above the rest and by enough that the above data demonstrates a consistent effect. That effect is seen most strongly in the cases of propyne and 2-butyne, which have the same property shared by none of the other compounds with three or more carbons, and that is that the geometry of the carbon chain is linear. This will certainly make a difference for packing molecules in the solid state and the average intermolecular distances being shorter (leading to strengthened dispersion forces) in the liquid state. This also leads to four carbon terminal linear chains in 2-pentyne and 2-hexyne, leading both to BP(2-pentyne) > BP(1-pentyne) and to BP(2-hexyne) > BP(1-hexyne), their three carbon terminal liner chain counterparts. 3-hexyne also has a four carbon linear section but it is internal and so not as beneficial to packing, shown by BP(2-hexyne) > BP(3-hexyne), but it is still helpful relative to intermolecular interactions for a three carbon linear section, hence BP(3-hexyne) > BP(1-hexyne).
I emphasise that I don't think an exam asking about these would be fair (except for trends like methane < ethane < propane < ...). The syllabus covers intermolecular interactions and bonding in an essentially qualitative manner and while that allows / supports many inferences about relative BPs, there remains a vast scope for quantitative study to understand multiple interacting facets acting collectively. Thus, if a question asked for a prediction of the BP of 1-pentyne and pentane and the student predicted pentane would be higher due to greater dispersion forces, even though the order is the reverse, I don't see a basis on which it could be reasonable to expect a student to know this or to deduce it from syllabus content. Similarly, whilst I might think it fair to ask which has the higher BP out of CaO and KF, I would not consider it fair to ask about NaCl v. KBr.
tyyyyy