Organic

Question 1

3 rules for resonance structures

Answer 1

Must have same molecular formula
Must have same total number of electrons
Must have same atoms bonded together

Question 2

3 legal moves of resonance

Answer 2

π bond -> lone pair
lone pair -> π bond
π bond –> π bond

Question 3

What are constitutional isomers?

Answer 3

Isomers with different connectives

Question 4

What are meta, para and ortho structures?

Answer 4

Apply to a benzene ring with 2 groups attached
meta - groups are 2 carbons away
para - groups are on opposite sides
ortho - groups are next to each other

Question 5

What are conformational isomers?

Answer 5

Can be converted into each other by rotation around a single bond
eg anti and syn structures

Question 6

What are enantiomers?

Answer 6

Non-superimposable mirror images of each other

Question 7

What are diastereoisomers?

Answer 7

Non-superimposable, not mirror images of each other

eg E/Z isomers, chiral molecules

Question 8

What are configurational isomers?

Answer 8

Involves the breaking of bonds to convert between the 2

2 types: diastereoisomers, enantiomers

Question 9

What are the 2 types of stereoisomers?

Answer 9

Configurational

Conformational

Question 10

What is a conjugated system?

Answer 10

Has 2 or more C=C bonds alternating with C-C bonds

Question 11

More conjugation leads to…

Answer 11

Smaller HOMO-LUMO gap

Question 12

For a reaction to take place, molecules must:

Answer 12

Overcome their electronic repulsion by charge attraction and/or orbital overlap

Have orbitals of appropriate energy to react - a filled orbital on the nucleophile and an empty orbital on the electrophile

Approach each other such that these orbitals can overlap to form a bonding interaction

Question 13

Nucleophiles typically…

Answer 13

Have a non-bonding lone pair of electrons (high energy HOMO)
eg ammonia, amines, water,

Generally, the higher the pKa of AH, the better A- is as a nucleophile

Question 14

Nucleophiles donate electrons from…

Answer 14

Available, high-energy orbitals from:
a lone pair
a negative charge
a double bond
a σ bond to an electropositive atom

Question 15

Electrophiles are…

Answer 15

Neutral or positively charged species with an empty atomic orbital or a low-energy antibonding orbital that can easily accept electrons

Question 16

What makes a good electrophile?

Answer 16

Has a low-energy anti bonding orbital associated with an electronegative atom
e.g. Having a double or single bond to an electronegative atom

Question 17

Electrophiles accept atoms into…

Answer 17

Empty low-energy orbitals represented by:
a positive charge representing an empty orbital
a neutral molecule with an empty p orbital
a double bond to an electronegative element
a single bond to an electronegative atom

Question 18

What makes a stable anion?

Answer 18

Having the negative charge on an electronegative atom or by spreading the negative charge over several atoms

Question 19

SN1 reaction

Answer 19

• Unimolecular
• Good LG needed
• Must be able to form a stable carbocation intermediate
• Nu independent
• Polar and protic solvent to stabilise ions

Question 20

SN2 reaction

Answer 20

• Biomolecular
• Good LG needed
• Sterically unhindered substrates
• Good Nu needed
• Solvent: not protic, but polar to dissolve Nu

Question 21

What molecules undergo an SN2 reaction?

Answer 21

CH3X > primary > secondary > tertiary (doesn’t react)

Question 22

What molecules undergo an SN1 reaction?

Answer 22

tertiary > secondary > primary (doesn’t react) > CH3+ (doesn’t react)

Question 23

What does the strength of an acid depend on?

Answer 23

The stability of the conjugate base

- the more stable the conjugate base, the stronger the acid

Question 24

What makes a good LG?

Answer 24

Weak bases - lower pKa means a better leaving group

Question 25

3 ways of making esters from alcohols

Answer 25

- acid anhydride + pyridine - acid chloride + pyridine - carboxylic acid + acid catalyst e.g. HCl

Question 26

Order of reactivity of carbonyls

Answer 26

Acid chloride > Aldehyde > Ketone > Ester > Amide

Question 27

The R/S isomer has the groups in order of decreasing priority in which direction?

Answer 27

R clockwise | S anticlockwise

Question 28

Stereochemistry of SN1 and SN2 reaction

Answer 28

SN1 usually produces a racemic mixture | SN2 inverts the carbon being attacked

Question 29

What are hard nucleophiles?

Answer 29

Small, charged, basic, low-energy HOMO, like to attack C=O, e.g. RO-, NH2-, MeLi

Question 30

What are soft nucleophiles?

Answer 30

Large, neutral, not basic, high-energy HOMO, like to attack saturated carbon, e.g. RS-, I-, R3P

Question 31

Which structure is more stable, staggered or eclipsed?

Answer 31

Staggered

Question 32

What conditions favour elimination over substitution?

Answer 32

High temperatures | Nucleophiles that are strong bases and bulky

Question 33

What conditions favour E2 reaction over E1?

Answer 33

High base concentration and strong bases favour E2

Question 34

E1 reaction must...

Answer 34

Must form stable carbocation

Question 35

Regio and stereo selectivity of E1 reaction

Answer 35

E isomers are favoured over Z | More stable alkenes are favoured

Question 36

Regio and stereo selectivity of E2 reaction

Answer 36

E2 reactions favour more stable alkenes, but become more regioselective for the less substituted alkene with more hindered bases

Question 37

What can never be a leaving group for E2?

Answer 37

OH-

Question 38

Order of steps for E1, E2, E1cB

Answer 38

E1 - leaving group first, then deprotonation E2 - deprotonation and leaving group simultaneous E1cB - deprotonation first, then leaving group

Question 39

Reducing agents

Answer 39

NaBH4 - will only reduce aldehydes and ketones | LiAlH4 - will reduce all carbonyls

Question 40

Reagents for making an aldehyde from an alcohol

Answer 40

CrO3 (can over oxidise) or PCC and DCM or DMSO, (COCl)2, Et3N and DCM

Question 41

Enolate systems tautomorise between...

Answer 41

Enol (alkene alcohol) and ketone

Question 42

Markovnikov's rule

Answer 42

For addition of HX across a double bond, the hydrogen will attach to the carbon with the most hydrogens on already

Question 43

Stereochemistry of adding Br2 to an alkene

Answer 43

Only anti product is formed (Brs on opposite sides) | Brs sticking up either side, with R groups dashed and wedged

Question 44

Stereochemistry for adding H2 to an alkene

Answer 44

Only syn product is formed (Hs on same side)

Question 45

Stereochemistry of adding HBr to an alkene

Answer 45

An equal mixture of anti and syn products are formed

Question 46

Alcohol + ketone -->

Answer 46

Acetal | where C=O was, turns into O-R (alcohol is ROH) going out both sides

Question 47

Ketone + amine -->

Answer 47

Imine | C=O goes to C=NR (amine is RNH2)

Question 48

Explain uses of enolates in organic chemistry

Answer 48

- Explain what they are - Relative reactivity / stability - How to make them - Alkylations - Condensations (aldol and claisen) - Michael reactions

Question 49

Rough pKa of carbonyls

Answer 49

``` Carboxylic acid - 5 Nitro - 9 Amide - 15 Alcohol - 17 Aldehyde - 17 Ketone - 20 Ester - 25 Nitrile - 25 Amine - 30 Alkane - 50 ```

Question 50

Definition of a chiral molecule

Answer 50

Is not superimposable on its mirror image (lack of plane of symmetry or centre of symmetry)

Question 51

How to separate a mixture of entiomers

Answer 51

React with another chiral molecule to form diastereoisomers, which have different physical properties e.g. boiling point