orgo question

[predentgal]

Advertisement - Members don't see this ad

A molecule has the molecular formula C20H16O. How many double bonds at most could this molecule contain?

The answer is 13. The explanation says it has something to do with degrees of unsaturation but it's not that clear. Any thoughts?

 

[UCB05]

A molecule has the molecular formula C20H16O. How many double bonds at most could this molecule contain?

The answer is 13. The explanation says it has something to do with degrees of unsaturation but it's not that clear. Any thoughts?

a completely saturated hydrocarbon comes in the form CxH(2x+2). Oxygen, if you remember, does not affect the ratio, as an oxygen takes away a H- but also contributes one. A 20-C saturated hydrocarbon should have 42 hydrogens. Instead it has 16, indicating a 26-hydrogen deficiency. Since each double bond decreases H's by 2, you have 13 double bonds.

 

[herkulease]

Yes this is degrees of unsaturation.

degrees of unsaturation is 2N + 2. N = # of Carbons. This tells you how many
hydrogens you need.

So we have 20 Carbons, plug it in we find we need 42 hydrogens.

But we only have 16. thus we lack 26. 26/2 = 13 degrees of unsaturation.

Why 2? if you form a ring or a double bond between carbons each carbon of those carbons will force to hold 1 less hydrogen each. thus 2.

So now that you know you have 13 degrees of unsaturation you can determine what you could have.

a Ring or double bond = 1 degree of unsaturation
a 3 triple bound = 2 degrees of unsaturation.

So with 13 degrees of unsaturation I could have 13 double bonds, 13 rings, 6 triple bonds and 1 double bond. You get the picture. You'll use NMR, mass spec to help determine the actual structure.

Oxygen is ignored in calculating degrees of unsaturation. Halogens are counted as if they were hydrogens. Nitrogens results in you need 1 more hydrogen for each nitrogen you have.

 

[predentgal]

In response to UCB do you always reference degrees of unsaturation to a saturated carbon? For example, if the question asked how manytriple bonds are in the molecule above would we still compute the number of hydrogens in a saturated compund and take the difference form the amount of H's we're given. So a triple bond would be 2 elements of unsaturation and would correspond to a maximum amount of 6.5triple bonds. Is that right?

And in response to hercules in the formula 2n + 2n = number of carbons whats n?

 

[UCB05]

In response to UCB do you always reference degrees of unsaturation to a saturated carbon? For example, if the question asked how manytriple bonds are in the molecule above would we still compute the number of hydrogens in a saturated compund and take the difference form the amount of H's we're given. So a triple bond would be 2 elements of unsaturation and would correspond to a maximum amount of 6.5triple bonds. Is that right?

And in response to hercules in the formula 2n + 2n = number of carbons whats n?

correct. a double bond counts as one degree of unsaturation. a triple bond counts as two degrees of unsaturation. You wouldn't necessarily say maximum of 6.5 triple bonds though. You could have at most 6... maybe 6 triple bonds and a double bond, 6 triple bonds and a ring, etc.

Herk means the formula is "2n + 2" and n = number of carbons. A completely saturated hydrocarbon with n carbons will have 2n+2 hydrogens.

 

[eunguyen]

The degree of unsaturation (also known as the index of hydrogen deficiency (IHD) or rings plus double bonds[1]) formula is used in organic chemistry to help draw chemical structures.

http://colapret.cm.utexas.edu/courses/IHD.pdf