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Organic Chemistry

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23 Jul 2009 10:41:13 IST

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Question 1:Is there any shortcut, or formula which enable's one to find out the exact number of stru

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Question 1:Is there any shortcut, or formula which enable's one to find out the exact number of structural isomers of an organic compound, when the number is as high as 36 or more? It's not possible (especially during a competitive exam) to draw all the possible structures on a rough space....

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djdylan2000. bits goa eee

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23 Jul 2009 11:14:30 IST

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AFAIK there is no formula for structural, formulae are only there for optical/geometrical

in entrance exams, they wont ask with number of C-atoms as large as 36!!!

Mr MindFreak

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23 Jul 2009 12:15:33 IST

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@djdylan2000 : he's rite !!

if you are having problem then check it here ..

http://www.scribd.com/doc/2903780/How-to-find-no-of-structural-isomers-

Aanusha Ghosh

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23 Jul 2009 14:57:27 IST

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36 is not the nmber of carbon atoms, but the number of possible isomers. And the .pdf document that I was asked to read still told me to draw all the structures... It's not possible in such a short time to draw all the 36 structures!

ADARSH SINHA

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23 Jul 2009 23:35:40 IST

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dere is no such formula........bt u cn make it short first find da degree of unsaturation......by dis u vl get da no. of double n triple bonds n possibility of rings in da compound............n den proceed......ny other query....post here.......

Divya Jyoti

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23 Jul 2009 23:45:04 IST

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questions where 12-15 isomers are involved are frequently asked.. so just get used to quickly calculating every type.

Aanusha Ghosh

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24 Jul 2009 12:20:44 IST

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How does one find out the degree of unsaturation? No one has taught me this procedure!

Akshay

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24 Jul 2009 12:47:31 IST

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The degree of unsaturation of a compound is half the number of hydrogen atoms which have to be added to its molecular formula so as to obtain its corresponding alkane.

Eg Ethene C₂H₄

2 H atoms needed to make it Ethane

Hence Degree of Unsaturation=2/2=1

Similarly Ethyne has two degrees of unsaturation

In order to find the degree of unsaturation of compounds containing Chlorine,Nitrogen,Oxygen,etc.

1)For a compound containing chlorine

Replace the chlorine atom in the formula by H atom and proceed as above using the new formula obtained.

2)Oxygen

Simply remove the atom from the formula and proceed as given above, applying the above method to the new formula

3)Nitrogen

Remove the N atom and ONE H atom from the formula and then apply the given algo. to the new molecular formula.

Aanusha Ghosh

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24 Jul 2009 12:57:41 IST

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Thank you very much! Now I understand how to find out the degree of unsaturation of a compund. Will you please tell me how finding out the degree of unsaturation helps one to calculate the number of possible structural isomers?

Akshay

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24 Jul 2009 13:11:36 IST

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Ya, because you can deduce from the molecular formula.

A double bond is equivalent to one degree of unsaturation and a triple bond is equivalent to two degrees of unsaturation.

A cyclic ring is equivalent to one degree of unsaturation

So from the molecular formula, you can degree of unsaturation and thus determine the possible structures having the formula.

E.g.

Suppose a compound has 4 degrees of unsaturation.

So the molecule is likely to have one of the following in its structure:

1)Two triple bonds

2)Four double bonds

3)Four rings

4)A triple bond,two double bonds

5)Two double bonds,two rings

6)Three rings, one double bond

7)Three double bonds, one ring

8)One triple bond,two rings.

Therefore you can determine the various structural isomers.

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