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Discussion in 'MCAT Study Question Q&A' started by Deepa100, Jun 15, 2008.
Is it because alcohol has a proton it can lose? Also, why is alcohol more polar than a ketone?
An alcohol is more polar than a ketone because it can engage in hydrogen bonding with more molecules of itself or with polar solvents. A ketone can hydrogen bond with other polar molecules with acidic hydrogens, but it cannot hydrogen bond with itself.
Alcohols have a different type of reactivity than ketones, and I'm not certain what you mean when you say that alcohols are more reactive. Ketones tend to react by nucleophilic addition/elimination reactions, while alcohols exhibit other kinds of chemistry, such as eliminations and substitutions.
Also, I believe the carbonyl bond is shorter (pi bond) than the C-O bond in alcohol, so the dipole moment would be smaller too, since it's a function of length.
I have seen a line in a AAMC test solution somewhere that stated that alcohols are more reactive. I forgot the context though, I will have to go back and look.
I don't think that's right; oxygen has 2 lone pairs on both the hydroxyl group and the carbonyl group
Alcohols are more reactive because the -OH can be made into a great leaving group via protonation. Turning a R-OH into R-OH2+ makes the R more susceptible to nucleophilic attack.
R-OH + HBr --> R-OH2+ + Br- --> R-Br + H2O
This can be done with HI and HCl (to a lesser degree) as well.
Ketones however, even when protonated, is not a leaving group. A protonated ketone will only allow the carbonyl carbon to be susceptible to nucleophilic attack, turning it into an alcohol.
Note however, these are just general rules. A complex tertiary alcohol will be LESS reactive than a simple acetone.