The presence of 11-cis retinal, C20H28O, in cells in the eye is important for vision.
The structure of retinal includes an aldehyde group, a cyclohexene ring and a long aliphatic side
chain, in which a carbon-carbon double bond exists between carbons numbered 11 and 12.
Which pair of statements about 11-cis retinal could be correct?
5 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side
5 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on opposite sides side
6 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side
6 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on opposite side
I automatically knew that the R1/R2 hydrocarbon groups had to be on the same side because it's a cis compound, which eliminates two of the answers, but then how do you figure out the number of C=C double bonds with only that information and the molecular formula?
5 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side
To determine the number of C=C double bonds in 11-cis retinal, you can start by analyzing its molecular formula, C20H28O.
First, subtract the number of hydrogen atoms (H) in the formula from the total number of atoms (C, O, H). In this case, C20H28O contains 48 hydrogen atoms (20 carbon atoms x 2 hydrogen atoms per carbon atom + 28 hydrogen atoms from the hydrogen atoms in the formula).
Next, divide the resulting number by 2 to account for each double bond requiring the elimination of two hydrogen atoms. In the case of 11-cis retinal, 48 ÷ 2 = 24, which is the total number of double bonds in the molecule.
Now, you can consider the options provided:
- If 11-cis retinal has 5 C=C bonds, it contradicts the information obtained from the molecular formula (24 C=C bonds).
- If 11-cis retinal has 6 C=C bonds, it is consistent with the information obtained from the molecular formula (24 C=C bonds).
Therefore, the correct pair of statements about 11-cis retinal is: it has 6 C=C bonds, and the R1/R2 hydrocarbon groups around the C=C bond between carbons 11 and 12 are on the same side.