Chem 1B Module 3 & 4

Diastereomers

Not mirror images; different compounds

Enantiomer

mirror images

constitutional isomer

has the same atoms but in a different arrangement

Stereoisomer

Has the same order of attachment for atoms but different 3D shape

Chiral

No symmetry

Achiral

Has symmetry

Stereocentre

A carbon connected to to 4 different atoms/groups (therefore is chiral)

TRUE or FALSE: Enantiomers have the same physical and chemical properties.

TRUE

What 2 things do enantiomers differ in?

1. Rotate plane polarised light the opposite direction

2. Different interactions with other chiral molecules

What is a) a mixture of both enantiomers called and b) what property does it have?

a) racemate

b) does not rotate plane polarised light

Cahn-Ingold sequence rules

1. Assign numbers 1 (highest atomic number) - 4 (lowest atomic number) to the molecules attached to stereocentre

2. If the first atom is the same go to next in the chain, remembering double bonded atoms are equal to two of the same atom

3. Rotate the molecule so group 4 is pointed away

4. Assign R or S configuration based on rotation of groups 1-3

S configuration

groups 1-2-3 are rotated anticlockwise

R configuration

groups 1-2-3 are rotated clockwise

Formula for number of potential stereoisomers

2^{(no. stereocentres + double bonds that can show E/Z isomerism)}

Meso compound

Contains stereocentres, is achiral

What type of stereoisomerism will a) the pictured molecule have and b) what are the requirements for it?

a) geometric

b) have two different substituents each on both ends of the double bond

Why do sp2 (trigonal planar) carbons not experience optical isomerism?

The π bonds involved in the double bond require a lot of energy to break and therefore allow rotation

E configuration

Of the highest priority groups on either end of a double bond, one is ‘above‘ and the other is ‘below‘ the double bond (ie. are on opposite sides)

Z configuration

Higher priority groups on either end of a double bond are both ‘above’/’below’ the double bond (Hint: think of it like connecting the same priority groups by tracing the shape of the letter Z)

Alkene

Hydrocarbon with a carbon to carbon double bond

How can one test for the presence of alkenes?

Add to Br₂ solution. If the brown solution turns clear there are alkenes present

Hydrogenation

Add H₂ on either end of an alkene double bond

Halogenation

Add X₂ (eg. Br₂ , Cl₂) on either end of an alkene double bond

Hydration

Add H₂O on either end of an alkene double bond; H & OH groups result

Hydrohalogenation

Add H-X (eg. Br, Cl) on either end of an alkene double bond

Which addition reactions can a) give more than one product under b) what conditions?

a) hydration, hydrohalogenation

b) if H-OH / H-X are added across the double bond of a non-symmetrical alkene

What do the arrows in a reaction mechanism represent?

The movement of electrons (from bonds being broken/made)

Fill in the chart

Regioselective mechanism

Selectively generate only certain product

What is the direction of movement of electrons in a reaction?

From nucleophile to electrophile

Nucleophile

electron rich/dontaes electrons; —

Electrophile

electron poor/receives electrons; +

Where does the negative charge end up at the end of a reaction?

The more electronegative atom (eg. Br vs C)

Carbocation

An intermediate product of an alkene addition reaction, formed at the most substituted (more attached C) end of the double bond. This means the first addition will take place at the other end.

What are the features (3) of an intermediate carbocation that increase with substitution, making it a more favourable intermediate?

1. more stable (due to more even distribution of charge)

2. faster reaction

3. lower activation energy/ more energetically favourable to form

Why does BH₂ add with the opposite regiochemistry to that of 70% H2SO4?

The electronegativity of H is greater than that of Br therefore Br will react first and H reacts with the carbocation

Why does cyclopentene give trans isomer only?

The second electrophile can only attach from the other side due to steric hindrance

What happens if both chloride and bromide ions are available?

Both compounds with two Br and compounds with one Br and one Cl will be made

What intermediate compound does the addition of bromine result in?

Bromonium ion

TRUE or FALSE: Z alkanes are mores stable than E alkanes

FALSE

Describe 3 features of ALIPHATICS

1. Fat like

2. burn with blue flame

3. low C:H ratio

Describe 3 features of AROMATICS

1. sweet smell

2. sooty flame

3. high C:H ratio

In terms of its reaction mechanism what differentiates benzene from alkenes?

Has substitution rather than addition reactions

In the presence of what do hydrogenation reactions need to occur?

Metal catalyst

What is the difference between addition and substitution reactions?

Addition reactions add a group whereas substitution reactions directly replace an already existing group

Wheland intermediate

carbocation formed on a benzene in the first step of a substitution reaction

What are the b) two options for the second step of a reaction with a benzene and b) which is favoured and why?

a) addition of the nucleophile or loss of a proton/H⁺

b) loss of a proton as it maintains resonance therefore is more stable

How does resonance impact stability?

Increases stability by spreading out electron density/delocalizing electrons

What is the purpose of a solvent?

Provide a medium for the reactants to dissolve and the reaction to take place

What differentiates polar protic solvents from polar aprotic solvents?

Polar protic solvents have acidic OH group(s)

List 3 common a) polar protic solvents and b) polar aprotic solvents.

a) water, methanol, ethanol

b) DMSO, DMF, acetone

What do the mechanisms of nucleophilic substitutions depend on (3)?

• alkyl halide (aka. substrate)

• nucleophile

• reaction conditions

What formula describes the rate of a S_N1 reaction?

rate=k[substrate]

where K is the rate constant

Are S_N1 reactions favoured by polar protic or aprotic solvents?

polar protic

List, in order of reactivity, the connectivity of carbons that can undergo S_N1 reactions.

Tertiary > secondary (not preferred)

What happens if an S_N1 reaction is carried out on a substrate of defined configuration?

A mixture of enantiomers

What does substitution depend on?

The amount of carbons linked to the carbon of interest

What happens if the S_N1 reaction takes place at a stereocentre?

racemisation

How many steps in S_N1?

2

Describe the process of an S_N1 reaction

1. The C-X bond breaks, forming a carbocation intermediate

2. The nucleophile (n^-) attacks the C⁺

What determines if the C-X bond is easy to break in an S_N1 reaction?

Ease of breaking increases with atomic size (eg. I>Br)

What step is a) the rate determining step of an S_N1 reaction and b) what is its relative speed?

a) the first step

b) slow

Why does racemisation occur in an S_N1 reaction?

the nucleophile can attack either side of the p orbital

What is the equation that describes the rate of an S_N2 reaction?

rate=k[substrate][nucleophile^-]

where k is the rate constant

How many steps is an S_N2 reaction?

1

What describes the number of steps associated with an S_N2 reaction?

simultaneous bond breaking and making

What happens to the stereochemistry of a molecule that has undergone an S_N2 reaction?

gets inverted; R/S, E/Z

List, in order of reactivity, the connectivity of carbons that can undergo S_N2 reactions.

primary > secondary

Why do tertiary substrates not react under S_N2 reaction conditions?

Too much steric crowding around the molecule the nucleophile needs to attack

What does a good leaving group do?

Carries negative charge from substitution;

What are 2 examples of good leaving groups?

• atoms/molecules that stabilize a negative charge

• conjugate base (X ^-) of a strong acid

What does the pKa of an acid mean for substitution reactions?

The lower the pKa, the better the leaving group (increasing size)

What makes an acid a strong acid?

The H being bound to a larger molecule

TRUE or FALSE: Br is a better leaving group than Cl

TRUE

Tosylate

converts an alcohol leaving group OH^- from a bad to a good leaving group

Why is OH^- a bad leaving group?

It is the conjugate base of H₂O which is a weak acid

What makes a tosylate a good leaving group (2) ?

• it is the conjugate base of a strong acid

• The anion is resonance stabilised through the benzene ring

Do aromatic halides a) undergo S_N1 or S_N2 reactions and b) why

a) Neither; do not at all undergo substitution reactions

b) S_N1: carbocation difficult to form, S_N2: too much steric interference from the ring for an attack from the rear

Aldehyde

R’COH

Ketone

RC=OR

Carbonyl group

C=O

What is the polarity of a carbonyl group?

⁺C=O^-

O is more electronegative

Do aldehydes/ketones have high or low boiling points?

low

What happens if an aldehyde is made by oxidizing a primary alcohol?

The reaction will proceed to oxidize further to create a carboxylic acid

What colour is Cr₂O₇^2-?

orange

What colour is Cr³⁺?

green

What conditions must be met so primary alcohols are selectively oxidised to aldehydes?

non-aqueous

What type of alcohol cannot be oxidized?

tertiary

Why can tertiary alcohols not be oxidized?

Oxidation removes the H from the O (of the alcohol) and the adjacent carbon. Tertiary alcohols do not have a H attached to the adjacent carbon.

Grignard reagent

Prepared by redox reaction between an organic halide and magnesium metal (R-MgX). They are a source of nucleophilic carbon.

Imine

The product of the addition of a primary amine (RNH₂) to an aldehyde or ketone gives, after the elimination of water

Carboxylic acid

COOH

Ester

RCOOR’

Amide

RCONR’R”

pK= [?]

-log₁₀Ka

Henderson-Hasselbach equation

pKa= pH - log₁₀ [conjugate base]/[Acid]

TRUE or FALSE: amines are acidic

FALSE: amines are basic, amides are non-basic

Why aren’t amides basic?

1. The lone pair on nitrogen is extensively delocalised and so less available for donation

2. When the amide is protonated on nitrogen, there is no possibility for resonance stabilization

How can the formation of esters via an equilibrium reaction with water and an alcohol be be maximised?

• using an excess of alcohol

• removing the water as it forms

How can esters be made?

• direct synthesis

• using acid chloride