CHM 233 : Fall 2017 Quiz #8 - Answer Key

Transcription

1 M 233 : Fall 2017 Quiz #8 - Answer Key Question 1 Malkanes5 Which best describes the relationship between the following two structures? trans-1,3-diethylcyclohexane A structural iosmers B different structures and not isomers stereoisomers the same thing trans-1,3-diethylcyclohexane = R = cis-1,3-diethylcyclohexane

2 A B Question 2 M19l What is the energy difference between the two chair conformations of the following molecule? i-pr ELIPSING Interactions Energy / ~1.0 / ~1.4 / ~2.6 GAUE Interactions Energy / ~0.9 / ~0.95 i-pr / i-pr ~1.1 t-bu / t-bu ~2.7 draw the 2 chairs and determine the energy cost associated with the varous substituents. Remember that 1 axial group "cost" is roughly the same as two gauche interactions with a methyl group. on't forget the / gauche interaction on the RE chair. 1 group axial, energy cost = 2 x (/) = group axial, energy cost = 2 x (/) = 1.8 TTAL energy cost = 3.7 i-pr i-pr 1 i-pr group axial, energy cost = 2 x (/i-pr) = / gauche, energy cost = 0.95 TTAL energy cost = 3.15 IFFERENE = = 0.55

3 Question 3 M23h which of the provided compounds has this IR spectrum? broad centered ca peaks above 3000 must be to sp2 hybrid carbons sharp benzene peak at N 2 conjugated = < A B B would have 2 peaks due to N- at ca. 3300, and also a broad weak band around 3000 (not observed), and would also not have = etc would also have - (not observed), would not have benzene peak at ca would not have carboxylic acid - at ca cm -1

4 Question 4 M19i Which is the lowest energy conformation of the following structure? A B ELIPSING Interactions Energy / ~1.0 / ~1.4 / ~2.6 GAUE Interactions Energy / ~0.9 / ~0.95 i-pr / i-pr ~1.1 t-bu / t-bu ~2.7 A this is a conformation of a different molecule, TIS NE >>> B 1 axial, energy cost ~ (2 x 0.95) = axial, energy cost ~ (2 x 0.9) = 1.8 TTAL energy associated with substituents in B = axial, energy cost ~ (2 x 0.95) = / gauche interaction cost = 0.95 TTAL energy associated with substituents in B = 2.85 TUS IS LWER IN ENERGY TAN B this is a conformation of a different molecule, TIS NE >>>

5 Question 5 Mir3 which of the provided compounds has this IR spectrum? not an alcohol R- centered at ca broad centered at ca not an aldehyde because it also has the broad - signal = at ca A B

6 Question 6 M23a which of the provided compounds has this IR spectrum? broad - around 3300 sp2 - >3000 sharp benzene at A would not have benzene peak at 1600 and no sp2 - would have N- peaks at ca and also strong = at ca would have carboxylic acid - centered ca. 3000

7 Question 7 M23f Identify the structure with the following IR spectrum peaks ~3350 & 3200 probable -N2 =, ~1650 cm-1, probable amide N 2 N 2 A B would not have = stretch no broad - observed at ca no broad - observed at ca. 3000

8 Question 8 Malkanes2 Identify the Newman projection that represents the IGEST energy conformer for rotation around the 3-4 bond in 2,2,3,5,5-pentamethylhexane. You need to identify the Newman projection that has 3 at the FRNT. t-bu t-bu R = 2 () 3 R t-bu t-bu R = 2 () 3 R A ) start by drawing the structure t-bu t-bu B 3) identify the 3 groups that are attached to the atom at the front ( 3 ) and at the back ( 4 ) = t-bu ) identify the bond we are concerned with, and "look" with 3 at the front, as shown t-bu t-bu Answer = 4) the IGEST energy conformation will be a fully eclipsed, with the largest substituents (t-bu) eclipsing t-bu t-bu Answer A is the correct conformation, but with 4 in the front!!!! R = 2 () 3 R Answers B and look down the 2-3 and 3-2 bonds, respectively 2 3 R = 2 () 3 R