CHEM高手快入 (S5 CHEM 1條)

"Ethyl is electron-withdrawing" ?!?!
are you sure?
all alkyl groups are electron-donating!


my GUESS:
the strength of H-bond depends on the distance between two molecules.
butan-1-ol has longer alkyl group, which potentially poses a greater steric hindrance to incoming molecules. the steric repulsion keeps molecules apart, so strength is lower.

alkyl group of ethanol is shorther than butan-1-ol's (ethyl 2xC VS butyl 4xC).


2010-10-24 02:35:32 補充：
003: "van der Waal's force will increase and its effect will overcome some of the strength of hydrogen bond" ??
overcome? they don't weaken another; they're both attractive force.

2010-10-24 02:35:43 補充：
003: "the strength of hydrogen bond of butan-1-ol is greater than ethanol"
you DIDN'T read the question.

It is because strength of hydrogen bond is also affected by the alkyl group attached to the -OH group. If the size of alkyl group is greater, e.g. butyl group, in this case, the van der Waal's force will increase and its effect will overcome some of the strength of hydrogen bond, so the strength of hydrogen bond of butan-1-ol is greater than ethanol.

It is because ethyl group is a electron withdrawing group,
the greater positive inductive effect appeared in butan-1-ol
the oxygen atom in butan-1-ol will become less negative than in ethanol