Author Topic: question related to free radical mechanism  (Read 3087 times)

Offline ashwinkandel

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question related to free radical mechanism
« on: October 04, 2010, 05:00:38 pm »
25 When heated with chlorine, the hydrocarbon 2,2-dimethylbutane undergoes free radical
substitution.
In a propagation step the free radical X• is formed.
CH3CH2C(CH3)2CH3 + Cl? ? X? + HCl

How many different forms of X• are possible?
A 1
B 2
C 3
D 4

can someone tell me the answers with reason and reacction plz.

Offline ashwinkandel

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Re: question related to free radical mechanism
« Reply #1 on: October 04, 2010, 05:02:09 pm »
another question too..
27 Which isomer of C4H10O forms three alkenes on dehydration?
A butan-1-ol
B butan-2-ol
C 2-methylpropan-1-ol
D 2-methylpropan-2-ol

Offline Deadly_king

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Re: question related to free radical mechanism
« Reply #2 on: October 04, 2010, 06:26:34 pm »
another question too..
27 Which isomer of C4H10O forms three alkenes on dehydration?
A butan-1-ol
B butan-2-ol
C 2-methylpropan-1-ol
D 2-methylpropan-2-ol

Butan-1-ol will form only one alkene since the OH group is found on the first carbon atom. Hence upon dehydration the OH group and an adjacent H atom will condense to form one mole of water to form an alkene.

Butan-2-ol will form 2 alkenes since it has equal chance of producing an alkene with the hydrogen atom from adjacent H atoms from both sides(Either the first carbon atom or the third one). Its OH group is situated on the second carbon atom.

2-methylpropan-1-ol has one OH group on the first carbon atom and two methyl groups on the neighbouring carbon atom. However, since it is a primary alcohol, dehydration will result in the formation of only one alkene since the carbon atom containing the OH group is attached to only one other carbon atom.

2-methyl-propan-2-ol is a secondary alcohol. If you draw its structural formula, you'll realize that any alkene formed will be the same one due to different possible orientations. Hence it will form only one type of alkene.

For me, all the four answers given are inappropriate. But if I had to choose one, it will be butan-2-ol since it can form the most alkenes possible (2).

NOTE : All primary alcohols are first eliminated since they can form only one type of alkene upon dehydration.

Though I can't find any other worth explanation  :-\
« Last Edit: October 05, 2010, 12:12:46 pm by Deadly_king »

Offline Deadly_king

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Re: question related to free radical mechanism
« Reply #3 on: October 04, 2010, 06:34:26 pm »
25 When heated with chlorine, the hydrocarbon 2,2-dimethylbutane undergoes free radical
substitution.
In a propagation step the free radical X• is formed.
CH3CH2C(CH3)2CH3 + Cl? ? X? + HCl

How many different forms of X• are possible?
A 1
B 2
C 3
D 4

can someone tell me the answers with reason and reacction plz.

I believe it is 4

Free radical substitution reaction may occur on each carbon atoms of the butane group. This makes a total of 3 types.

Then one type for the methyl groups, in both cases it will be the same radical. Hence it is taken as only one type.

Try draw its displayed formula. It might help you to understand better and don't forget about the different orientations  ;)


Offline ashwinkandel

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Re: question related to free radical mechanism
« Reply #4 on: October 04, 2010, 06:48:10 pm »
this is the question from 2010 paper no 12. according to marking scheme the answer of question one which i asked is B and question two is C.

Offline Deadly_king

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Re: question related to free radical mechanism
« Reply #5 on: October 05, 2010, 06:52:05 am »
this is the question from 2010 paper no 12. according to marking scheme the answer of question one which i asked is B and question two is C.

Yeah.........the first question is C indeed.

Sorry for the mistake. I misunderstood the question.

When you draw the displayed formula you'll notice that the compound has 4 methyl groups(CH3) and one CH2 part. Free radical substitution mechanism involves the substitution of one H atom by a halogen atom.

So when one H atom is removed from the first CH3, one type of radical is formed and when one H atom is removed from the CH2 part, another type of radical is formed. Then another type is formed when one H atom from any 3 methyl groups on the third carbon atom. Due to orientations any radical formed with the methyl groups will be the same. This makes a total of 3 types.

Reactions :
1. CH3CH2C(CH3)2CH3 + Cl* ---> CH2*CH2C(CH3)2CH3 + HCl

2. CH3CH2C(CH3)2CH3 + Cl* ----> CH3CH*C(CH3)2CH3 + HCl

3. CH3CH2C(CH3)2CH3 + Cl* ---> CH3CH2C(CH3)2CH2* + HCl


By the way I modified my post for the second number  ;)
« Last Edit: October 05, 2010, 11:20:16 am by Deadly_king »

Offline ashwinkandel

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Re: question related to free radical mechanism
« Reply #6 on: October 05, 2010, 01:31:30 pm »
THANKS DEADLY_KING. U HAVE BECOME BY TEACHER. I UNDERSTOOD IT CLEARLY.

Offline Deadly_king

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Re: question related to free radical mechanism
« Reply #7 on: October 05, 2010, 02:10:07 pm »
THANKS DEADLY_KING. U HAVE BECOME BY TEACHER. I UNDERSTOOD IT CLEARLY.

OMG ::)

Me.......teacher ???

You must be joking dude :P

Anyway it's a great pleasure to help you ;)

Offline Vin

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Re: question related to free radical mechanism
« Reply #8 on: October 05, 2010, 03:06:54 pm »
OMG ::)

Me.......teacher ???

You must be joking dude :P

Anyway it's a great pleasure to help you ;)

Believe me man, you should seriously opt for the option, as a career. You'll earn gigabucks! :@ At least teachers here do. :P

Offline Deadly_king

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Re: question related to free radical mechanism
« Reply #9 on: October 05, 2010, 03:24:35 pm »
Believe me man, you should seriously opt for the option, as a career. You'll earn gigabucks! :@ At least teachers here do. :P
But what makes you think i'll be successful as a teacher ??? I am only sharing the knowledge that I acquired throughout my studies.

Yeah......I know teachers especially those giving tuitions earn a lot. But really, am not so interested in earning money.

Since i've been a kid I wanted to help those in need.........that's why I badly want to be a medical practitioner. In this way i'll be able to help the poor and even save lives.
I know this may be an ancient reason but I believe in that :)

Anyway thanks for the compliment :)
« Last Edit: October 05, 2010, 03:26:47 pm by Deadly_king »