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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: xstrae on April 20, 2007, 12:42:05 AM

Title: Carbenes: comprehension question
Post by: xstrae on April 20, 2007, 12:42:05 AM
Hi,

Can you please help me get started on these 3 questions?

1) Arrange the following carbenes in the increasing order of stability if they are
(a) singlet carbenes
(b) triplet carbenes

:CH2 , :CF2 , :CCl2

 2)

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg167.imageshack.us%2Fimg167%2F276%2Fcarblh9.png&hash=e32c6da799585606edccf2a87b3b48b428420491)

 Which of the following statements is/are correct about A and B if the carbene is singlet?
 
(A) They are diastereomers
(B) They are geometrical isomers
(C) They are enantiomers
(D) None of the above
 
3) The number of products formed in the reaction given in the previous question, if the carbene is triplet is
(A) 2
(B) 3
(C) 1
(D) 4
 
Title: Re: Carbenes: comprehension question
Post by: FeLiXe on April 21, 2007, 09:57:51 AM
1) carbenes are more stable if the substituents are good pi donors.
without pi-donors the carbene is a triplet because of Hund's rule. if you have pi-donors then the carbene is a singulet because the p-orbital is used to accept pi-electrons

2) a singlet reacts in one concerted step, no rotation is possible. you have to think about how the two isomeres are related depending on if the carbene comes from the front or back

3) with a triplet carbene you have an intermediate that freely rotates