IIT JEE , AIEEE Forum - Chemistry , Physical Chemistry

savvej

Which of the following are arranged in correct decreasing order of dipole moment?
options:
(1)CH₃Cl>CH₃F >CH₃Br>CH₃I
(2)CH₃F>CH₃Cl>CH₃B>CH₃I
(3)CH₃Cl>CH₃Br>CH₃I>CH₃F
(4)CH₃F>CH₃Br>CH₃I>CH₃F
THE ANS GIVEN IS OPTION (A ) BUT I CANT GET WHY IT IS THE RIGHT OPTION.
WHY IS OPTION 2 NOT CORRECT AND OPTION 1 CORRECT????

pannaguma

i dont think ans is A.
has to be B.

savvej

THIS IS A QUESTION FROM MCQ BY AK GOSH AND AHULIWALIA.
uptil now i havent found ans answer form this book as incorrect.so
plz tell me surely whether the ans is a or b???

priyesh

the ans. given is correct

i also had the same problem a few months back

see as as electronegativity decreses down the group hence polarity of C-X bond decreases and hence dipole moment decreases(provided effect of increase in bond length is not sufficient to cause any major effect)

hence by this the expected order is that of B)

but order is A) because the difference between the fluorine & chlorine atomic size is quite large as compared to the difference down the group(see Figure).So although C-F bond is more polar(hence more charge),but bond length of C-F is quite less than that of C-Cl bond.Hence bond length factor dominates in this case making C-Cl dipole moment more than C-F.

Hope you understood

Cheers!!!!!!!!!!!!!!!!!!!!!!!!!!!

aditi_g

but this is an exception case in electronegativity....
chlorine is more electronegative than fluorine as given in 11 th standard chem NCERT...
therefore the answer is A) the answer given is correct

Ashish

The answer is A.
As the electronegativity of the atom attached to the Carbon atom decreases, the polarity of the bond also decreases. Hence,

CCl > CBr >CI

This much is clear.

But CCl > CF!

Why?

Because although fluorine is more electronegative, due to the small size of the Fluorine atom the dipole moment, which is the product of the charge and the separation between the charges, comes out to be smaller than in the case of CCl.

tarinbansal

Aditi_g,
ur concept is wrong. U said that Cl is more electronegative than F. It is wrong. Infact F is the most electronegative element in the whole periodic table,leave alone Cl.

Perfect answer Priyesh.

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