ORGANIC OPTION -G. IB Core Option Objective Electrophilic Addition Reactions G.1.1 Describe and explain the electrophilic addition mechanisms of the reactions

ORGANIC OPTION -G

IB Core Option Objective

Electrophilic Addition Reactions• G.1.1 Describe and explain the

electrophilic addition mechanisms of the reactions of alkenes with halogens and hydrogen halides.

Terms

• Electrophile:

• Carbocation:

• Carbocation stabalization:

• Nucleophile: (Loves positives) Any molecule or ion that has a lone pair of electons

G.1.1 Describe and explain the electrophilic addition mechanisms of the reactions of alkenes with halogens

and hydrogen halides.

• Alkene double bond is attacked by an electrophile

C

H

H

H

Cl Cl

1-Chloroethane

Cl

H +

–Cl

C


• G.1.2 Predict and explain the formation of the major product in terms of the relative stabilities of the carbocations.

G.1.2 Predict and explain the formation of the major product in terms of the relative stabilities of the carbocations.

• Markovnikov’s Rule– H-X when added to a multiple bond molecule,

hydrogen will always add to the carbon with the most hydrogen's.

Primary Secondary Tertiary

Increasing stability

Carbocation stabilization• Iodine monochloride:

– Identify the more electronegative group– Is the molecule polar?– Draw the steps for how the product is formed, and

name the product.

Iodine monochloride + propene


Nucleophilic Addition Reactions• G.2.1 Describe, using equations, the addition

of hydrogen cyanide to aldehydes and ketones.

• G.2.2 Describe and explain the mechanism for the addition of hydrogen cyanide to aldehydes and ketones.

G.2.1 Describe, using equations, the addition of hydrogen cyanide to aldehydes and ketones.

• HCN + OH- H2O + CN- (Requires a base catalyst to make CN- which is a better nucleophile)

• Significance: Increases carbon chain length by one carbon

Cyanide ion + Propanone

G.2.1 Describe, using equations, the addition of hydrogen cyanide to aldehydes and ketones.

• Draw the VSEPR diagram for CN-

C

O

R R H

C

O

R

Which end is slightly positive and negative?

G.2.2 Describe and explain the mechanism for the addition of hydrogen cyanide to aldehydes and ketones.

• CH3CHO + HCN with OH-

• Get in partners, use the molecular model kits and make the structures for each.

• Play out the steps for this reaction in pairs.• While working the steps in 3D, show the steps

for this reaction on paper, showing the intermediate step and the product. Use curly arrows to show electron transfer.


• G.2.3 Describe, using equations, the hydrolysis of cyanohydrins to form carboxylic acids.

G.2.3 Describe, using equations, the hydrolysis of cyanohydrins to form carboxylic acids.

• The Nitrogen group can be removed by:– 1) Adding a base to produce carboxylic acid & NH3

– 2) By adding an acid to produce NH4+

CR N

Acid (H

+ ) Base (OH-)

NH3NH4+ CR

O

O-OHCR

O


Hydrolysis• Take the product you made from the previous

reaction (2-hydroxypropanenitrile) and react it with water and acid. What are your products?

• A: 2-hydroxypropanoic acid (lactic acid) and NH4

+.


O

H

H

C

H

H

H

CC

O

HO

O

H

C

H

H

C

H

H


• G.4.1 Describe, using equations, the reactions of 2,4-dinitrophenylhydrazine with aldehydes and ketones.

G.4.1 Describe, using equations, the reactions of 2,4-dinitrophenylhydrazine with aldehydes and ketones.

• Addition: 2,4 –dinitrophenylhydrazine acts as a nucleophile (non-bonding pair of electrons).

• Elimination: The two hydrogens bonded to the nitrogen will bond with the oxygen on a ketone or aldehyde, causing atoms to be eliminated to form water.

N N

H

H

HNO2

NO2


N N

H

H

HNO2

NO2 + Propanone

Significance: Creates an orange-yellow precipitate which can be melted to test what ketones or aldehydes are present in solution based on its MP.

Note: Products from these reactions are called 2,4-dinitrophenylhydrazones. You do not need to know how to name specific products for this reaction.

You don’t need to know every step!

http://upload.wikimedia.org/wikipedia/commons/0/01/DNPH-mechanism.svg



• G.3.1 Describe, using equations, the dehydration reactions of alcohols with phosphoric acid to form alkenes.

• G.3.2 Describe and explain the mechanism for the elimination of water from alcohols.

G.3.1 Describe, using equations, the dehydration reactions of alcohols with phosphoric acid to form alkenes.

• What would the equation be for the dehydration of ethanol?

• A: C2H5OH → C2H4 + H2O• What would be needed to protonate the

hydroxyl group? Is it a catalyst or part of the products?

• A: Strong acid, preferably phosphoric (V) acid, H3PO4. It is a catalyst, since a proton is donated back to regenerate the acid.

G.3.2 Describe and explain the mechanism for the elimination of water from alcohols.

Dehydration of alcohols to form alkenes using acids.

Butan-1-ol

Concentrated phosphoric acid

180 oC

Butan-2-ol

Concentrated phosphoric acid

180 oC

G.3.2 Describe and explain the mechanism for the elimination of water from alcohols.

Mechanism Butan-1-ol Phosphoric acid

PO3H2H O

Carbocation

Carbocation ButeneConjugate Base

IB Core Option Objective• G.5.1 Describe and explain the structure of

benzene using physical and chemical evidence.

• For physical evidence, include a comparison of carbon–carbon bond lengths in alkanes, alkenes and benzene, and the number of structural isomers with the formula C6H4X2.

• For chemical evidence, include a comparison of the enthalpies of hydrogenation of benzene, cyclohexene, 1,3 cyclohexadiene ‑and 1,3,5-cyclohexatriene, and the tendency of benzene to undergo substitution rather than addition reactions.

G.5.1 Describe and explain the structure of benzene using physical and chemical evidence.

What do you know?You already know some things about benzene.

Brainstorm with a partner what you already know! Think back to Topic 4 or 10!


• Arenes: Compounds that contain benzene ring.• How do we know: Three separate double bonds with

localized electrons or resonance structure with delocalized electrons?

OR

Arenes• Evidence:• 1) Will not react with bromine water

– Significance: There must be no double bonds• 2) Six equal bond lengths

– Significance: No alternating double/ single bonds as double bonds are shorter, single bonds are longer.

• 3) Benzene is more thermodynamically stable– Significance: Combustion of cyclohexatriene

should result in more energy released then predicted.

Br Br

Booya double bond I’m gonna react you

good!!What the??

Double bonds are shorter than

single!!!


4) There are no second structural isomers for 1,2 disubstituted benzene compounds. (There is only one)

5) Look at the study guide for the enthalpy of hydrogenation of cyclohexene. What would you expect if you hydrogenated two more double bonds, compared to what happens in benzene?


• G.5.2 Describe and explain the relative rates of hydrolysis of benzene compounds halogenated in the ring and in the side-chain.

• Only the reactions with the OH– ion will be assessed.

G.5.2 Describe and explain the relative rates of hydrolysis of benzene compounds halogenated in the ring and in the side-chain.

• Arenes can be halogenated directly on the benzene ring:

• Or they can be halogenated on a side chain to the ring:

Br

C Br

H

H

G.5.2 Describe and explain the relative rates of hydrolysis of benzene compounds halogenated in the ring and in the side-

chain.

• If it is on the side chain, it undergoes nucleophilic substitution just like a halogenoalkane. What is needed as a reagent and what would the mechanism be?

? + → ?

A: Aqueous NaOH, SN2

C Br

H

H

G.5.2 Describe and explain the relative rates of hydrolysis of benzene compounds halogenated in the ring and in the side-chain.

• If the halogen atom is directly attached, nucleophilic substitution does not occur, or it occurs very slowly.

• This is due to the dense cloud of electrons surrounding the arene ring, thus the nucleophile is repelled.

• It is also due to the stronger C-Br bond from the benzene than the C-Br bond in halogenoalkanes.

Br


• G.6.1 Outline the formation of Grignard reagents.

• G.6.2 Describe, using equations, the reactions of Grignard reagents with water, carbon dioxide, aldehydes and ketones.

G.6.1 Outline the formation of Grignard reagents.

• Using halides to create longer carbon chains by Nucleophilic addition, particularly using electron deficient carbons on ketones and aldehydes

• Grignard Reagents

Mg + R-X R-Mg-X

Carbon anion formed due to the electronegativity difference

between C-Mg

δ- δ+

G.6.2 Describe, using equations, the reactions of Grignard reagents with water, carbon dioxide, aldehydes and ketones.

R--Mg--Xδ- δ+

+

δ-

Mg--X

R

O

H

CH

H

H

C

H

C

H

--

–

H+

Acid removes remaining part of Grignard reagent in the intermediate step

H

Reactions and Products• 1) R-Mg-Cl + CO2 R-COOH

– Carbon dioxide Carboxylic acid

• 2) R-Mg-Cl + H2O R-H + Mg(OH)Cl– Ketone Alkane

• 3) R-Mg-Cl + C=O R-C-OH– Methanal Primary alcohol

• 4) R1-Mg-Cl + R2-CHO R1-CH(R2)-OH– Aldehyde Secondary alcohol

• 5) R1-Mg-Cl + R2-CO-R3 R1-C(R2R3)-OH– Ketone Tertiary alcohol

Questions

• Mg + 1-Chloroethane

• 1) Grignard + Water • 2) Grignard + Carbon dioxide • 3) Grignard + Methanal • 4) Grignard + Propanal • 5) Grignard + Propan-2-one • 6) Make the Grignard 2-Chlorobutane


• G.8.1 Describe and explain the acidic properties of phenol and substituted phenols in terms of bonding.

• Review: If an acid has a weak conjugate base, is it a strong or weak acid?

A: It is strong.

G.8.1 Describe and explain the acidic properties of phenol and substituted phenols in terms of bonding.

• Phenols are weakly acidic, but stronger than ethanol.

• This is because the negative charge on the conjugate base can be spread out over the entire benzene ring (resonance).

OH O-

O

Can you figure out the other resonance structures?

G.8.1 Describe and explain the acidic properties of phenol and substituted phenols in terms of bonding.

• Negative charge can be spread even further when nitro groups are added.

• They are electron withdrawing, leading to an even weaker conjugate base.

• Thus, 2,4,6-trinitrophenol is a strong acid.• A group that donates electrons, such as CH3, will make the

anion less stable (stronger conjugate base) and the acid weaker.

• Q: What would the order of pKa, from largest to smallest, be for ethanol, phenol, 2-methylphenol, and 2,4,6-trinitrophenol?

• A: Ethanol: pKa=16, 2-methylphenol pKa = 10.26, Phenol pKa=10.0, 2,4,6-trinitrophenol pKa=0.42


• G.8.2 Describe and explain the acidic properties of substituted carboxylic acids in terms of bonding.

G.8.2 Describe and explain the acidic properties of substituted carboxylic acids in terms of bonding.

• The conjugate base of carboxylic acids has two resonance forms.

• Adding more methyl groups will have a positive inductive effect, making the acid weaker.

• Adding electron withdrawing substituents such as chlorine will make the acid stronger.

C

O

O-

H C

O

O-

H C O

O-

H


• G.8.3 Compare and explain the relative basicities of ammonia and amines.

G.8.3 Compare and explain the relative basicities of ammonia and amines.

• Like ammonia, amines will act as weak bases when dissolved in water:

R-NH2 + H2O R-NH3 + OH-

• Alkyl amines are stronger bases than ammonia (positive inductive effect).

• The longer the alkyl group, the more basic the amine.

• The base strength increases from a primary amine to a tertiary amine.

G.8.3 Compare and explain the relative basicities of ammonia and amines.

• What would the equation be if ethylamine reacted with hydrochloric acid?

• A: C2H5NH2 + HCl → C2H5NH3+Cl-

(ethylammonium chloride)

• What would happen if you were to react the ethylammonium chloride with warm sodium hydroxide?

• A: C2H5NH3+Cl- + NaOH → C2H5NH2 + NaCl + H2O

Advanced Question • How can you make 2,3 dimethyl-pent-3-ol

– Using only Propan-1-ol and Butan-1-ol• Press for hint # 1• Press for hint # 2• Press for hint # 3• Press for hint # 4• Press for hint # 5• Press for hint # 6• Press for hint # 7

OHOH

MgClCl

OH

Documents

ORGANIC OPTION -G. IB Core Option Objective Electrophilic Addition Reactions G.1.1 Describe and explain the electrophilic addition mechanisms of the reactions