Strength of hydrogen bonding in phenol or methanol?

Question

I wanted to know whether hydrogen bonding is stronger in phenol or methanol. I saw on Wikipedia about the enthalpies of hydrogen bonding in various cases. but could not find the answer to this question of mine. I also want to know whether the hydrogen bond strength depends upon the carbon atom to which oxygen is attached.

Answer 1

Hi there,

Great question! This is a rather tricky question, but the short answer is that methanol (and unhindered alcohols in general) participate in Stronger hydrogen bonding than phenol (and many phenolic compounds).

A sort of rough estimate or "shortcut" you could use to compare the H-bonding strengths for these molecules is comparing the BPs of the corresponding alkane (or arene) that lacks the OH group required for H-bonding. For example, methane (CH4) has a BP of -161.5 C and methanol (CH3OH) has a BP of 64.7 C: That is over a 200 C difference! Whereas benzene (C6H6) has a BP of 80.1 C and phenol (C6H5OH) has a BP of 181.7 C: This difference in BP is only about 100 C (roughly). Since the main difference in intermolecular forces for methanol vs. methane or phenol vs. benzene is the H-bonding ability, the BP differences can approximate the strength of the H-bonding. Stronger H-bonding would make it much harder to boil a compound since you would have to break these intermolecular bonds to form a vapor. Since 200 C > 100 C, the H-bonding in methanol must indeed be stronger due to the higher BP difference.

The following is a more in depth view that uses some more complicated concepts:

I would say the more "classical" way approach this problem is to consider the aspects of the compound(s) of interest that are known to contribute to weaker or stronger H-bonds. The biggest aspects in this case to consider would be the acidity of the H-bond donor (compound "donating" an H+) and the basicity of the H-bond acceptor (compound with Oxygen lone pairs "accepting" the H+, aka. forming the intermolecular bond with this H+).

Consider Methanol (CH3OH): methanol has a pKa of ~16 and so is only somewhat acidic (with the conjugate base, (CH3O)^- . Also, it is somewhat basic and able to form (CH3OH2)⁺ when acid is present. It is important to note that H-bonding does not really involve these species directly (these are more extreme scenarios) because only partial bonds are formed and broken. However, it is useful to consider them when trying to discern H-bond strength for a compound or set of compounds. With only a somewhat small tendency to give up an H+ or accept an H+, we can assume the H-bonding is of intermediate strength for methanol.

Now the problem gets a bit tricky with phenol. Phenol is a little more acidic than methanol (phenol pKa ~10), and so is more willing to be an H-bond donor ("donate" an H+), but the oxygen in neutral (uncharged) phenol has one lone pair that exists in a p-orbital and participates in resonance with the arene ring system. This means that the oxygen contains a formal positive charge in said resonance structure and only has one available lone pair (a much less nucleophilic sp2-hybridized lone pair) available for accepting an H+. It turns out the extreme lack of basicity informs the H-bond strength more than the higher acidity when it comes to phenol, essentially making its intermolecular hydrogen bonding weaker than that of methanol.

This may be a bit more than you bargained for haha but the info is there if you care to read it! My answers can be as short or as long as desired for your particular needs (this goes for everyone)!

-Michael F.