F.5 有機化學問題

1. The hydrogen atom is attached with carbon atoms but not on the oxygen atom. So no hydrogen bond can be formed. (You should note that the definition of criteria hydrogen bond formation, which the hydrogen atom should attach to a very electronegative atom, e.g. F, O and N)

2. I don't agree with Wai Ho's answer. He states that each amine molecule can form 3 hydrogen bonds with other amine molecules. But it is wrong. It should be two instead. Because each amine molecule has only one lone pair of electrons on nitrogen atom, which can only form one hydrogen bond with other amine molecule. And similarly one of the hydrogen atoms on amine molecule can only form one hydrogen bond with the lone pair of electrons on nitrogen atom. So the maximum number of hydrogen bond formed is 2 only.

The case is different from the number of hydrogen bonds formed between amine and water. Because water has two lones in O atom, which can formed two hydrogen bonds with hydrogen atoms in amine, while the lone pair of electrons in nitrogen atom can form one hydrogen bond with the hydrogen atom in water molecule.

3. He didn't answer the question. You asked about why can't nitrogen atom in amide molecule can also form hydrogen atom with other amide molecule. Same explanation as above. Because the two lone pairs of electrons in oxygen atom can only form two hydrogen bonds with two hydrogen atoms attached with nitrogen atom of other amide. And the two hydrogen atoms attached with the amide molecule can also form two hydrogen bonds can also form two hydrogen bonds with the two lone pairs of electrons in oxygen atom with other amide molecule. Thus, the max. number should be 4 only. However, the lone pairs of electrons of nitrogen atom is neglected because firstly, there is no more hydrogen atoms provided for it to form hydrogen bond and secondly, the nitrogen atom is less electronegative than oxygen atom. So the case of nitrogen atom is not counted.

2011-06-05 15:21:07 補充：
Because of the word limit, question 4 is answered as follows:
Totally there are two hydrogen atoms formed. First, one of the lone pairs of electrons can form one hydrogen bond with the hydrogen atom. Second, the hydrogen atom attached to the oxygen atom can also form one hydrogen bond.

2011-06-05 15:34:49 補充：
A little adding of Q3:
The nitrogen atom is less electronegative than oxygen atom. So the influence of hydrogen bond formed between the lone pair of electrons of nitrogen atom and hydrogen atom is less than between the lone pair of electrons of oxygen atom and hydrogen atom.

2011-06-05 15:34:55 補充：
In this case, the molecule would like to select oxygen atom to form hydrogen bond rather than nitrogen atom.

2011-06-05 17:57:48 補充：
Consider the whole population of amine molecules
Each amine molecule has 1 lone pair of electrons in N atom, if there are x amine molecules, then there are x lone pairs of electrons.
Then each amine molecule has 2 hydrogen atoms, if there are x amine molecules, then there are 2x hydrogen atoms.

2011-06-05 17:59:23 補充：
However, only 1 lone pairs of electrons of electronegative atom can form 1 hydrogen bonds with 1 hydrogen atom attached to a electronegative atom. So x lone pairs of electrons can only form x hydrogen bonds with x hydrogen atoms.

2011-06-05 17:59:53 補充：
That means what I said not enough of lone pairs of an electronegative nitrogen atom to form hydrogen bond.

2011-06-05 18:05:35 補充：
This is a similar situation for alcohol. There are 2 lone pairs of electrons in O atom but just 1 hydrogen for each alcohol molecule. The maximum number of hydrogen bonds formed is just 2 only.

1. in ester there are only C-O bond, C-H bond , O-C bond
there are no bond that can form hydrogen bond with other ester

2.係, 如果以maximun 黎睇

3. amide: C(=O)-N-H2
Oxygen有2個lone pair 所以計maximun有2個
有2個N-H bond 再加多2個

所以maximun係4個with other amide molecule.

5.如果又計maximun，甘就3個
不過通常我地話係一個,由於1H