Organic Chemistry - organic.. SM

13

Avik ·2010-02-18 07:43:24

Waise, success magnet mein bahut answers wrong hote hain naa...i've heard. [4]

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Pritish Chakraborty ·2010-03-19 00:54:03

aieeee, if the solvent is protic, comparing between O and S, S having larger size will tear more easily through the envelope of protic molecules surrounding it. This is similar to order F < Cl < Br < I in protic solvents because I has largest size.
S has to be the more nucleophilic atom.
When we compare different nucleophiles in which nucleophilic atom is same(here it is O^-), nucleophilicity order will follow basicity order. Thus the order I have mentioned.

How does a protic solvent make a larger molecule less nucleophilic??? Balki oxygen atoms will attract more protic solvent molecules towards itself and find it harder to tear through that envelope because of its smaller size.

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aieeee ·2010-03-18 11:52:47

1) lets see which among the following is least stable after addition of H+ ion :

a) R-NH₃⁺ is stable as electron releasing group R stabilises the positive charge.

b) R-C⁺-OH-NH₂ is also stable as lone pairs of O and N stabilise the +ve charge.

c) R-C⁺-OH-R is also stable as the two alkyl groups stabilise the positive charge.

d) R-C⁺=NH , a vinylic type structure is formed which is unstable.

Thus , nitriles are least basic.

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aieeee ·2010-03-18 11:29:55

First of all , the medium is aqueous i.e. protic solvent (H₂O)
Its obvious that CH₃OH is the least nucleophilic , while CH₃S^- is the next one as its the larger molecule than rest. thus , a protic solvent makes a larger molecule less nucleophilic.

now , from the rest three , THE BASICITY ORDER hs to be first found out.

carboxylate ion < phenoxide ion < hydroxide ion

NOTE : If the solvent is protic , then the basicity and nucleophilicity are inversely related

thus, the nucleophilicity order is : carboxylate ion > phenoxide ion > hydroxide ion

so, the order goes as : carboxylate ion > phenoxide ion > hydroxide ion > CH₃S^- > methanol

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JOHNCENA IS BACK ·2010-03-17 13:39:45

for 4.my logic says,it is:::(b),(d)

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JOHNCENA IS BACK ·2010-03-17 12:06:32

guys my logic says it is::::

4>5>2>3>1

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106

Asish Mahapatra ·2010-03-17 09:37:32

not sure...

it was aieeee's doubt....

he says this is the order given in Bruice which he feels is wrong

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39

Pritish Chakraborty ·2010-03-17 01:01:11

Asish can you confirm Q5?

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Pritish Chakraborty ·2010-02-20 20:32:58

Q5 - Main concept : If the nucleophilic atoms in different molecules are the same, nucleophilicity order follows basicity order. You can compare phenoxide, carboxylate, etc that way. Methyl sulfide anion will be strongest nucleophile I guess...

For the others, add a proton to compare.
I - CH₃O⁺H₂
II - Phenol
III - Carboxylic acid
IV - H₂O

From these, it is clear I is the most acidic, it can give a proton up most easily. Following next is phenol, as phenoxide ion is heavily stabilised by resonance. Then comes carboxylic acid, which is lightly stabilised by resonance. Water is the weakest acid here.
So acidity order is -:
I > II > III > IV
Basicity order will be ulta
IV > III > II > I

So the actual order is CH₃S^- > OH^- > Carboxylate > Phenoxide > Methyl alcohol
This order is also verified by the fact that methyl alcohol has no formal charge on it and is hence the weakest base.

Sulphur is more nucleophilic here because of its larger size which allows it to tear more easily through the envelope of water molecules present in solvent, which surround it.