IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

Utsav55

Following are the groups attached to the Benzene ring.

How can u tell whether the group is having -I/+I and -R/+R effect?

-N(CH3)3 positive charge on N

-OCH3

-CH $\doteq$ CH-CHO where $\doteq$ is double bond.

-NHCOCH3

Please give the explanation on how u found the effects of each.

Thanks

karthik2007

For inductive effect, if the atom attached to the Carbon atom has greater electronegativity than carbon, it will show I- effect, and vice versa.
For resonance effect, an atom having a lone pair of electrons will generally show +R effect, and an atom having no lone pair of electrons will show -R effect (eg Carbon itself)

tarinbansal

Karthik's explanation is correct.
So by his explanation,

-N(CH3)3 positive charge on N---> Very Strong -I
-OCH3---> -I, +R
-CH=CH-CHO---> Slight -I, -R
-NHCOCH3 --> -I, -R

chinmay_saxena01

yup karthik explanation is absolutely correct....

mukundmadhav

NHCOCH3 has +R effect.

Rest is ok

Utsav55

an atom having no lone pair of electrons will show -R effect (eg Carbon itself)
Is this thing always true??

No lone pair, then -R, plz explain -R effect in this condition.

Thanks for the rest.

mukundmadhav

See +R and -R effect basically refer to the concentration of negative charge in the delocalised electrons in the benzene ring. If a group is increasing the negative charge, it is said to have +R effect. If it is decreasing, it is said to have -R effect. Atoms like, O, N, X have lone pair of electrons left over after complete bonding has taken place. So these are called electron rich species. These can contribute their lone pair of electrons to the benzene ring. So they are +R groups(Note that for halogens -I effect is stronger than +R effect)
On the other hand, Groups like N(CH3)3 +, -CHO, NO2 have a positive charge(unit positive charge or delta +) on the central atom. So they have a tendency to withdraw electrons from the benzene ring
So they are called -R groups

tarinbansal

@mukundmadhav,

He has given in the question that in N(CH3)3, there is positive charge on N, means no lone pair. So IT WONT SHOW +R EFFECT. IT WILL SHOW ONLY -I.

And also in the case of NHCOCH3, dont forget that the lone pair on N is also in continuous conjugation with C=O and as O is more elevtronegative than N, it will withdraw the lone pair of N which in turn will withdraw the electron density from the ring. so IT WILL SHOW -R EFFECT.

mukundmadhav

N(CH3)3 will not.. That was an error on my part
But NHCOCH3 will. You can check it out in any textbook.. In fact, it's +R effect is comparable to OCH3's
N has very large donating tendency as compared to O. So extended conjugation won't matter..

mukundmadhav

http://books.google.co.in/books?id=ega5c11VHvkC&pg=PA348&lpg=PA348&dq=NHCOCH3+activating+group+benzene&source=web&ots=tM1i0Qfyts&sig=jm6fsvwhPxg9zYDhBxmdvj2Rmzo&hl=en

tarinbansal

OOoops, Sorry.
U were right. NHCOR is +R.

Utsav55

oh! getting confused.

If +R is the effect which increases the e- density then what is +I and -I effect?

-I is supposed to be decreasing the e- density, right??

mukundmadhav

No dude.. - and + don't refer to the charge.. Only the tendency to give or take away electrons..

+I increases negative charge.

Utsav55

Quick conclusion:

+I increases -ve charge
-I decreses -ve charge
+R resonance of lone pair or which donates its lone pair
-R which doesn't use its lone pair?? check this plz

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