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

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marked as duplicate by ringo, Todd Minehardt, Jon Custer, Freddy, bon Apr 24 '16 at 19:15

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    $\begingroup$ Because there is always a heavier alkane with equal number of carbon atoms than one with an odd number of carbon atoms. $\endgroup$ – aventurin Apr 23 '16 at 17:26
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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

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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

enter image description here

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.

enter image description here

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