C5 Conjugation

Question 1

state properties of conjugation

Answer 1

• enhanced stability
• changes in structure (planarity)
• different reactivity to non-conjugated systems (more stable so less reactive)
• characteristic spectroscopic behaviour (absorption of UV / visible light to create colour)

Question 2

how do double bonds relate to conjugation?

Answer 2

• alternating double and single bonds are said to be conjugated
• conjugation starts and end with a double bond so extra methyl groups are not included

Question 3

other than double bonds, where else can conjugation occur?

Answer 3

• a lone pair adjacent to a double bond can sometimes, but not always, be conjugated (eg. lone pair on nitrogen in an amide is conjugated with the carbonyl)
• lone pairs on oxygen are also considered to be conjugated to the carbonyl group in an ester bond

Question 4

what shape do alternating single and double bonds produce and why? what else is produced?

Answer 4

• produce a continuous run of p orbitals
• run of p orbitals enforces conjugation and therefore planarity
• extended pi bond is produced through entirety of molecule

Question 5

how does an extended pi bond cause planarity?

Answer 5

• electrons are delocalised throughout the entire pi system above and below the plane of the atoms
• causes planarity in the molecule (won’t allow rotation)

Question 6

why can’t you rotate a conjugated run of single and double bonds?

Answer 6

• overlap of p orbitals would break and prevent conjugation
• molecule wants to be conjugated as it makes it more stable so it stays as planar as possible

Question 7

what effect does conjugation have on structure, eg. in single bonds?

Answer 7

gives them partial double bond character so the central C-C bond is shorter than you would expect

Question 8

describe conjugation in amides

Answer 8

• nitrogen in amides can change its hybridisation to sp2 and put its lone pair into a p orbital so it can partake in conjugation
• this lowers the energy of the molecule and stabilises it
• forms extended pi bond over 3 atoms (O=C-N)

Question 9

why are both amides and alternating single and double carbon bonds both conjugated?

Answer 9

• they both have a continuous, uninterrupted run of 3 or more atoms, each with an unhybridised p orbital

Question 10

what hybridisation states are required for conjugation and why?

Answer 10

• sp or sp2
• sp3 can’t take part
• an unhybridised p orbital is required

Question 11

what is the smallest conjugated system?

Answer 11

• a double bond with an adjacent p orbital
• orbital overlap occurs and gives an extended pi bond where electrons are delocalised

Question 12

what can conjugated systems do in regards to light?

Answer 12

• compounds containing conjugated multiple bonds absorb light at wavelengths longer than 200nm (in the visible)

Question 13

how does the wavelength of light absorbed change when the length of the conjugated system changes?

Answer 13

• the longer he conjugated system, the longer the wavelength of the light absorbed

Question 14

describe the electromagnetic spectrum in relation to conjugation

Answer 14

• UV has a lower wavelength and higher energy
• visible light is in the middle
• IR has a higher wavelength and lower energy

Question 15

what is the key concept that must be remembered when determining what colour a conjugated molecule would appear?

Answer 15

• if a substance absorbs visible light, the colour it appears is complementary to the colour it absorbs
• colours are diametrically opposite on colour wheel
• if absorbs green, appears red etc.