Nucleophilic subsititution or addition?

[msk]

I am really confused with nucleophilic substituyion and addition stuff and also about electrophilic stuff...please if some one could explain it......

 

[hassam]

http://www.chemguide.co.uk/mechmenu.html#top
u ll get all the mechanisms explained here......

 

[wahab305]

just remember one thing that as alkenes has double bond their double bond can be easily ruptured by any electrophile because of the high electron density of the pi bonds in C=C bond thus this addition occur and usually halogens are added so there is no sorts of any confusion in this where as for alkanes with all single bond they undergo substation because the carbon already has 4 bonds thus there could be no addition there since they undergo free radical substitution reactions

 

[DragonCub]

Well, a shortcut to remember the two:
There are C=C DOUBLE bonds before reaction but none after ------> addition
No change to the C=C bonds or totally free of it ------> substitution

There must be a double bond, that is two 'vacancies' for adding anything. Right?

 

[msk]

ok i get substituition and addition stuff but is there an easy concept of determining wheather it is nucleophilic or electrophilic?

 

[MAVtKnmJ]

Nucleophilic reactions occur when a Nucleophile or a Negatively charged species attacks e.g OH-, Cl - etc
Electrophilic reactions occur when an Electrophile or a Positively charged species attacks e.g H+, Br+, H3O+

 

[msk]

NO2+ is positive but goes nuclephilic substitution?.....

 

[Rockstar3143]

well.... its a real shortcut..
alkenes:- always electrophlic addition of things like Br2, HBr, etc.
aldehydes:- always nucleophilic addition of HCN..
alkylhalides:-always nuclephilic substition of KCN (alc) or KCN(aq).
benzene ring:- electrophilic substitution.

 

[msk]

benzene ring electrophilic substitution?.... what about nitration of benzene?

 

[MAVtKnmJ]

Nitration of benzene (it has an electron cloud) is by NO2+ (it is an electrophile), thus it gives an electrophilic reaction.
Have a look at the mechanism for clarification:

http://pubs.acs.org/doi/abs/10.1021/jo991538u

 

[fireman]

Thanks