2 Chemistry questions - need some help

[J_o_S]

got it. thanks guys

 

[Dr.Brain]

1) (i) ----> Break the double bond of propene heterolytically and see where the carbocation is more stable , Br- will attack on the more stable carbocation (if the reaction is being undertaken in absence of peroxide)

(ii) This process is known as esterification , look for the mechanism for esterification in your book and sort out the reaction yourself and show your work.



(i)Electrophilic Substitution

(ii) Esterification.

BJ

 

[siddharth]

J_o_s: First of all, its 1-propene. Refer the IUPAC nomenclature rules.

Then, look at the structure of propene. The carbon-carbon double bond is rich in electrons. And HBr is an acid. Therefore, where will the H+ ion attack? What's the intermediate formed? Have you learned about carbocations and their rearrangments?Then what will the Br- do?
Also, have you learned about Markovnikov rule.?


Dr Brain: it's an Electrophilic Addition reaction.

 

[siddharth]

For a) I was thinking I could just show that both reaction mechanisms add up to the same reaction.

for b)i) I think the reaction rate would triple, for b)ii) I don't think the reaction rate would change.

not sure about part c

Can anyone point me in the right direction?

thanks

parts a,b(i) and b(ii) are right
For part c, use the fact that the rate of the reaction depends only on the concentration of I-.
Also the rate determining step is the slowest step of a reaction.

 

[Dr.Brain]

J_o_s: First of all, its 1-propene. Refer the IUPAC nomenclature rules.

Then, look at the structure of propene. The carbon-carbon double bond is rich in electrons. And HBr is an acid. Therefore, where will the H+ ion attack? What's the intermediate formed? Have you learned about carbocations and their rearrangments?Then what will the Br- do?
Also, have you learned about Markovnikov rule.?


Dr Brain: it's an Electrophilic Addition reaction.

siddharth , this is a two stage process ,first the double bond is broken and then Br- attacks.In addition processes, the attack and leaving processes both take place simultaneously.

 

[Dr.Brain]

For 2) part ,

i) The first part is simple.You have been given the step-wise mechanism for a single reaction , when you add them up , they both give the same reaction.The difference between both of these mechanisms is that of 'slow' steps which are the 'rate-determining-steps'.

Here (2) is the correct mechanism .Search for the reason. Maybe you know something about how a reaction progresses , from slow to fast? ...or medium,slow and fast?

 

[siddharth]

siddharth , this is a two stage process ,first the double bond is broken and then Br- attacks.In addition processes, the attack and leaving processes both take place simultaneously.

A reaction in which the substrate and the reagent add up to form a product is called addition reaction.
In this case, HBr is the reagent and propene is the substrate.
Where is the leaving process in the addition reaction??

 

[Dr.Brain]

A reaction in which the substrate and the reagent add up to form a product is called addition reaction.
In this case, HBr is the reagent and propene is the substrate.
Where is the leaving process in the addition reaction?

Leaving process is in substitution and not in addition , have you heard of
$S_N - 1$ and $S_N - 2$ ... ? , the same happens here.

BJ

 

[siddharth]

Leaving process is in substitution and not in addition , have you heard of
$S_N - 1$ and $S_N - 2$ ... ? , the same happens here.

BJ

Let's go through the definitions again

Addition reaction:
A reaction in which the substrate and the reagent add up to form a product is called addition reaction.

Substitution reaction:
A reaction in which one group or atom is replaced by another is called a substitution reaction.


Also, under addition reactions, we have
(i) Electrophilic addition reaction:
Here the attacking species is an electrophile.(Example: Br+ or H+) An example would be
CH2=CH2 + Br2 ----------> CH2Br-CH2Br

(ii) Nucleophilic addition reaction:
The attacking species is a nucleophile(example: CN-). An example would be
the cyanohydrin reaction
RCHO + HCN ---------> RCH(OH)(CN)



And under substitution reactions, we have
(i) Free radical substitution
Example:
CH4 + Cl2------------> CH3Cl + HCl

(ii) Nucleophilic substitution
Here one nucleophile is displaced by another.
Example:
R-X + OH- ----------> R-OH + X-

This reaction can proceed under two major mechanisms, namely the SN1 and SN2 mechanisms. SN1 is a first order reaction and follows first order kinetics whereas SN2 is a second order reaction and follows second order kinetics.
Apart from this there is also the SNi mechanism or Substitution Nucleophilic Internal mechanism.

In the above reaction the attacking species is OH- and the leaving group is X-

(iii) Electrophilic (aromatic) substitution.

Example:
ArH + RCl ---------> ArR + HCl.

Here the attacking species is the electrophile E+ ( In the above example R+).
and the leaving species is H+.

So, what does the reaction
CH3-CH=CH2 + HBr ---------> CH3-CHBr-CH3 fit under?
Obviously it is an addition reaction.
Also, the attacking species is H+ which is an electrophile. Therefore the reaction is an electrophilic addition reaction.
Hope that clears things up.


Apart from this there is a third class of organic reactions which are the elimination reactions. But that's another story.

 

[Dr.Brain]

(ii) Nucleophilic substitution
Here one nucleophile is displaced by another.
Example:
R-X + OH- ----------> R-OH + X-

This reaction can proceed under two major mechanisms, namely the SN1 and SN2 mechanisms. SN1 is a first order reaction and follows first order kinetics whereas SN2 is a second order reaction and follows second order kinetics.
Apart from this there is also the SNi mechanism or Substitution Nucleophilic Internal mechanism.

In the above reaction the attacking species is OH- and the leaving group is X-

You said it all. In the above reaction if it had been a complex substrate other than propene, the carbocation re-arrangement would have taken place, and after the $C^+$
rearranges , the Br- attacks just like in SN-1 reactions.This is a two step mechanism.A case of substitution.

BJ

 

[siddharth]

So what if it is a two step mechanism? The criterion for substitution reaction is NOT a two step mechanism. In fact, SN2 reaction is via a SINGLE STEP mechanism but it is a substitution reaction.
The criterion for a substitution reaction is that something must get substituted. If you say CH3-CH=CH2 + HBr ---------> CH3-CHBr-CH3 is a substitution reaction, what is getting substituted?

 

[Dr.Brain]

Do you know the mechanism for $CH_3 - CH = CH_2$ with $Br_2$, it follows a bromonium ion mechanism , another example that a cation is involved in a substitution.Same in above.

The difference between Substitution and Addition is that in substitution , the reagent ion substitutes the most favourable position depending on where the carbocation stability is max and in addition is an enforced version in which one atom is 'pushed' out , rather being allowed to stabilise.

 

[siddharth]

A substitution reaction is when one group REPLACES another. In the reaction between Propene and Br2 and the reaction between Propene and HBr, there is NO substitution.
In other words, there is no replacement of one group by another group.

What happens is that the substrate ADDS with the reagent to give the final product.
If you notice in these reactions, there is no leaving group. Since the substrate ADDS with the reagent, it is an example of and ADDITION reaction.