Resonance

Question 1

which bond is shorter: single or double?

Answer 1

double

pull the electrons closer together which decreases the length of the bond

Question 2

more than one Lewis structure (resonance)

Answer 2

both equivalent and equally correct

only differ in the formal arrangement of electrons - not position of atoms!

better picture of electron distribution obtained by taking average of both Lewis structures

Question 3

symbol to represent the two Lewis structures

Answer 3

double headed arrow between them

Question 4

name given to the alternative Lewis structures (more than one for a molecule)

Answer 4

resonance hybrid

Question 5

energy in a resonance hybrid compared to the individual Lewis structures

Answer 5

lower energy than either of the individual Lewis structures that contribute to it

Question 6

when does resonance arise?

Answer 6

when two or more Lewis structures may be written for a molecule with different arrangements of electrons but identical arrangements of atoms

Question 7

when can resonance structures not be equivalent?

Answer 7

neutral and dipolar form (e.g. methanal)

resonance hybrid is a weighted average with a larger contribution from the neutral resonance form than from the dipolar one

Question 8

how to represent changes in electronic rearrangement (show electron movement)

Answer 8

curly arrows

Question 9

why isn’t a dotted line used instead of a curly arrow?

Answer 9

unclear how many electrons are represented by a series of dots - can’t electron count

Question 10

when are resonance hybrid’s valid?

Answer 10

don’t break octet rule

atoms haven’t changed position

Question 11

how can acid strength be modified?

Answer 11

mesomeric effects in either the acid or conjugate base

Question 12

what makes a resonance structure more stable?

Answer 12

if charge is delocalised - exist on more than one atom

more delocalisation = more stability

Question 13

why are carboxylic acids stronger acids than alcohols?

Answer 13

carboxylate anion is resonance stabilised

lone pair of e- on O- conjugates with pi bond in C=O in carboxylate

• ve charge shareed between 2 O atoms
• ve charge on alkoxide is localised on 1 atom only

Question 14

tautomers

Answer 14

rapidly intercovertible isomers which differ in the location of a mobile atom (usually hydrogen)

involves real structures in equilibrium with different geometries and different arrangements of nuclei and electrons