Elevated boiling points in alkanes with even number of carbons

Question

Why do alkanes with even number of carbon atoms have greater boiling points than those with odd number of carbon atoms?

Answer 1

There is not really any difference between the boiling points of even and odd carbon alkanes. For single-chain alkanes the boiling point just goes up as a smooth curve versus chain length.

The melting points do show a small oscillating component with the even chains being higher. If we imagine the chains in their idealized zig-zag configuration, which would be relatively favorable for the solid state, we find that the even-length chains have a center of inversion which makes them fit better than odd-length ones. This effect fades, however, with increasing chain length.

Reference:

https://en.wikipedia.org/wiki/Alkane

Answer 2

I think you asked your question poorly. Here is data from Wikipedia. Most of the BP's were given as ranges. I rounded some and some I took the average. But in all cases as the number of carbons increases, the BP increases.

 alkane        #Carbons   B.P. (°C)
 methane          1       −161.49
 ethane           2        −88.5
 propane          3        −42.2
 n-butane         4          0
 n-pentane        5         36
 n-hexane         6         69
 n-heptane        7         98
 n-octane         8        125
 nonane           9        151
 decane          10        174
 undecane        11        195
 dodecane        12        216

I also fit the data to a 3rd order polynomial.

BP=-2.262586E+002 + 7.517034E+001*C - 5.350278E+000*C^2 + 1.810153E-001*C^3

    alkane    #Carbon  BP(data)   BP(fit)  error 
    methane     1        -161.5   -156.3    5.2
    ethane      2         -88.5    -95.9   -7.4
    propane     3         -42.2    -44.0   -1.8
    n-butane    4           0.0      0.4    0.4
    n-pentane   5          36.0     38.5    2.5
    n-hexane    6          69.0     71.3    2.3
    n-heptane   7          98.0     99.9    1.9
    n-octane    8         125.0    125.4    0.4
    nonane      9         151.0    148.9   -2.1
    decane     10         174.0    171.4   -2.6
    undecane   11         195.0    194.2   -0.8
    dodecane   12         216.0    218.1    2.1

Here is a plot of the residuals. As Oscar Lanzi indicated there is no evidence of an even-odd trend.