Sellord
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I just came up with this question in a past paper. It is talking about C3H7I

"(iii) Halogenoalkane P forms the yellow precipitate faster than the halogenoalkane Q. Draw a displayed formula for halogenoalkane P."

Answer is CH3-CHI-CH3

May someone explain why the answer is a secondary halogenoalkane? Basically why are secondary halogenoalkanes more reactive than primary...

Thanks.
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charco
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(Original post by Sellord)
I just came up with this question in a past paper. It is talking about C3H7I

"(iii) Halogenoalkane P forms the yellow precipitate faster than the halogenoalkane Q. Draw a displayed formula for halogenoalkane P."

Answer is CH3-CHI-CH3

May someone explain why the answer is a secondary halogenoalkane? Basically why are secondary halogenoalkanes more reactive than primary...

Thanks.
Because the intermediate is stabilised by the +I effect of two alkyl groups.

This stabilisation means that the activation energy for its formation is less and, as this step is the slow, rate determining, step of the mechanism, the reaction is quicker
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username913907
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The mechanism may be Sn2 like. (Look it up)
It may form a secondary carbon centered cation which are more stable than primary ones as the alkyl groups can inductively donate electrons to the cation. In less advanced language the electrons from the C-H bonds in the R groups can be partially donated to the cation stabilising the charge.
This makes tertiary carbocation more stable than secondary ones, so you should see a tertiary haloalakane react faster.
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illusionz
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(Original post by JMaydom)
The mechanism may be Sn2 like. (Look it up)
It may form a secondary carbon centered cation which are more stable than primary ones as the alkyl groups can inductively donate electrons to the cation. In less advanced language the electrons from the C-H bonds in the R groups can be partially donated to the cation stabilising the charge.
This makes tertiary carbocation more stable than secondary ones, so you should see a tertiary haloalakane react faster.
Think you mean Sn1 like
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MarshmallowBob
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(Original post by Sellord)
I just came up with this question in a past paper. It is talking about C3H7I

"(iii) Halogenoalkane P forms the yellow precipitate faster than the halogenoalkane Q. Draw a displayed formula for halogenoalkane P."

Answer is CH3-CHI-CH3

May someone explain why the answer is a secondary halogenoalkane? Basically why are secondary halogenoalkanes more reactive than primary...

Thanks.
its due to the fact that its a tertiary halide.
Thats the simple answer. lol. Because its a tertiary halide the mechanisim is different.
Hope i helped.
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charco
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(Original post by MarshmallowBob)
its due to the fact that its a tertiary halide.
Thats the simple answer. lol. Because its a tertiary halide the mechanisim is different.
Hope i helped.
Not really, Bob, considering that it's a secondary haloalkane ...
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Sellord
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(Original post by charco)
Because the intermediate is stabilised by the +I effect of two alkyl groups.

This stabilisation means that the activation energy for its formation is less and, as this step is the slow, rate determining, step of the mechanism, the reaction is quicker
(Original post by JMaydom)
The mechanism may be Sn2 like. (Look it up)
It may form a secondary carbon centered cation which are more stable than primary ones as the alkyl groups can inductively donate electrons to the cation. In less advanced language the electrons from the C-H bonds in the R groups can be partially donated to the cation stabilising the charge.
This makes tertiary carbocation more stable than secondary ones, so you should see a tertiary haloalakane react faster.
I do understand this theory; the fact that the methyl groups in the side donate electron density and stabilize the center, and therefore is a more stable halogenoalkane.

However, what I dont get is why this stability in secondary halogenoalkanes makes them more reactive with hot aqueous silver nitrate, and therefore the formation of the yellow ppt faster?
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MarshmallowBob
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(Original post by charco)
Not really, Bob, considering that it's a secondary haloalkane ...
:O your right. lol.

"-The tertiary halide produces a precipitate almost instantly.

-The secondary halide gives a slight precipitate after a few seconds. The precipitate thickens up with time.

-The primary halide may take considerably longer to produce a precipitate"

http://www.chemguide.co.uk/organicpr...nes/agno3.html

lol i hope that helps now.
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Sellord
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(Original post by MarshmallowBob)
:O your right. lol.

"-The tertiary halide produces a precipitate almost instantly.

-The secondary halide gives a slight precipitate after a few seconds. The precipitate thickens up with time.

-The primary halide may take considerably longer to produce a precipitate"

http://www.chemguide.co.uk/organicpr...nes/agno3.html

lol i hope that helps now.
Thanks for the link man, chemguide is always useful.

Then I suppose that you just need to know this? Since chemguide states: "It is more difficult to explain the reason for this, because it needs a fairly intimate knowledge of the mechanisms involved in the reactions." I'm doing edexcel AS chemistry.
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MarshmallowBob
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(Original post by Sellord)
Thanks for the link man, chemguide is always useful.

Then I suppose that you just need to know this? Since chemguide states: "It is more difficult to explain the reason for this, because it needs a fairly intimate knowledge of the mechanisms involved in the reactions." I'm doing edexcel AS chemistry.
I've come across Sn1 and Sn2 once, but it was in a Cambridge AS study guide.
But I doubt youll need to know about it.
lol, btw good luck 9 days to the exam.
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username913907
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Your right, Sn1.... Sorry. Do look up the mechanism though, it a very basic mechanism. Ag is considered Halophillic, so will take away halogens if available. Useful in a variety of syntheses as you can remove the halide ion from an equilibrium. (ie Hydrolysing a halide)
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mrunalidarji
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(Original post by Sellord)
I just came up with this question in a past paper. It is talking about C3H7I

"(iii) Halogenoalkane P forms the yellow precipitate faster than the halogenoalkane Q. Draw a displayed formula for halogenoalkane P."

Answer is CH3-CHI-CH3

May someone explain why the answer is a secondary halogenoalkane? Basically why are secondary halogenoalkanes more reactive than primary...

Thanks.
Hey! I have come across the same question but unfortunately but i dont understand the question that followed the question you asked! how does reacting the halogenoalkane p which was c3H7I with hot aqueous silver nitrate form an alcohol?
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