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?
To determine the number of carbon-carbon double bonds (C=C bonds) in 11-cis retinal, you can use the information given in the molecular formula.
The molecular formula of 11-cis retinal, C20H28O, tells us that it contains 20 carbon atoms, 28 hydrogen atoms, and 1 oxygen atom. By subtracting the number of hydrogen and oxygen atoms from the total number of atoms, we can determine the number of carbon atoms that are part of the carbon chain.
20 - (28 + 1) = 20 - 29 = -9
Since we cannot have a negative number of carbon atoms, we need to account for the double bonds in the structure. Each carbon-carbon double bond results in the loss of two hydrogen atoms, as double bonds replace single bonds. By dividing the deficit in carbon atoms by 2, we can determine the number of double bonds.
-9 / 2 = -4.5
Since we cannot have a fractional number of double bonds, we take the absolute value of the result, which is 4.5. However, the number of double bonds in a molecule must be a whole number. So, we round the result to the nearest whole number, yielding 5 C=C bonds.
Therefore, the correct statement about 11-cis retinal is: 5 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side.