CHEM-103: BASIC ORGANIC CHEMISTRY

transcript

LECTURE 6

Dr Ali El-Agamey

CHEM-103:

BASIC ORGANIC CHEMISTRY

DAMIETTA UNIVERSITY

٢Chapter 10 ٢

Reduction of Carbonyl

• Reduction of aldehyde yields 1º alcohol.

• Reduction of ketone yields 2º alcohol.

• Reagents:– Sodium borohydride, NaBH4

– Lithium aluminum hydride, LiAlH4

– Raney nickel

٣Chapter 10 ٣

Sodium Borohydride

• NaBH4 is a source of hydrides (H-)

• Only reacts with carbonyl of aldehyde or ketone, not with carbonyls of esters or carboxylic acids.

٤Chapter 10 ٤

Lithium Aluminum Hydride

• LiAlH4 is source of hydrides (H-)

• Stronger reducing agent than sodium borohydride, but dangerous to work with.

• Reduces ketones and aldehydes into the corresponding alcohol.

• Converts esters and carboxylic acids to 1ºalcohols.

٥Chapter 10 ٥

Reduction with LiAlH4

٦Chapter 10 ٦

Reducing Agents

• NaBH4 can reduce aldehydes and ketones but not esters and carboxylic acids.

• LiAlH4 is a stronger reducing agent and will reduce all carbonyls.

٧Chapter 10 ٧

Catalytic Hydrogenation

• Raney nickel is a hydrogen rich nickel powder that is more reactive than Pd or Pt catalysts.

• This reaction is not commonly used because it will also reduce double and triple bonds that may be present in the molecule.

• Hydride reagents (NaBH4 and LiAlH4) are more selective so they are used more frequently for carbonyl reductions.

٨Chapter 10 ٨

Catalytic Hydrogenation

Reagent used:

Haloform reaction

Sodium hypohalite

Potassium hypohalite

Mechanism:

Oxidation

Halogenation

Cleavage

Iodoform(Yellow ppt)

Haloform reaction can convert an alcohol to a carboxylic acid with one less carbon atom.

Haloform reaction

Homework: Which of the following compounds will give a positive iodoform test?

Homework

Ketones and Aldehydes

Organic Chemistry, 7th EditionL. G. Wade, Jr.

(1) Ketones: Parent name is alkanone

Choose the longest continuous chain containing the carbonyl group.

In cyclic ketones the carbonyl carbon atom is assigned the number 1.

(2) Aldehydes: Parent name is alkanal

An aldehyde carbon is at the end of a chain, so it is number 1.

For cycloalkanes with aldehyde substituent, the suffix -carbaldehyde is used.

IUPAC nomenclature

Nomenclature of aldehydes and ketones

Functional group Name as substituent

Aldehydes Formyl

Ketones Oxo

Alcohols Hydroxy

Amines Amino

Ethers Alkoxy

Halides Halo

IUPAC nomenclature

Nomenclature of aldehydes and ketones

١٧١٧

Examples

3-methyl-2-butanone 4-hydroxy-3-methyl-2-butanone

CH3 CH3 C

CH2OH1

3-bromocyclohexanone

CH3 CH2 CH

CH2 C H

3-methylpentanal1

3-methyl-4-oxopentanal

CH3 C CH

CH2 C H

Write the IUPAC name of the following compounds

Homework: Write the IUPAC name of the following compounds

Ketones and Aldehydes

Organic Chemistry, 7th EditionL. G. Wade, Jr.

٢١Chapter 18 ٢١

Carbonyl Structure

• Carbon is sp2 hybridized.

• C═O bond is shorter, stronger, and more polar than C═C bond in alkenes.

٢٢Chapter 18 ٢٢

Boiling Points

• Ketones and aldehydes are more polar, so they have a higher boiling point than comparable alkanes or ethers.

• They cannot hydrogen-bond to each other, so their boiling point is lower than comparable alcohol.

٢٣Chapter 18 ٢٣

Solubility of Ketones and Aldehydes

• Good solvent for alcohols.

• Lone pair of electrons on oxygen of carbonyl can accept a hydrogen bond from O—H or N—H.

• Acetone and acetaldehyde are miscible in water.

R and R1 may be alkyl or aryl

Preparation of ketones

CH3CH2CH2COCl

٢٦Chapter 18 ٢٦

• A Grignard or organolithium reagent can attack the carbon of the nitrile.

• The imine is then hydrolyzed to form a ketone.

٢٧Chapter 18 ٢٧

Nucleophilic Addition

• A strong nucleophile attacks the carbonyl carbon, forming an alkoxide ion that is then protonated.

• Aldehydes are more reactive than ketones.

Reactions of aldehydes and ketones

(1) Clemmensen reduction (2) Wolff-Kishner reduction

٢٩Chapter 18 ٢٩

Formation of Imines

• Ammonia or a primary amine reacts with a ketoneor an aldehyde to form an imine.

• Imines are nitrogen analogues of ketones and aldehydes with a C═N bond in place of the carbonyl group.

• Optimum pH is around 4.5

٣٠Chapter 18 ٣٠

Other Condensations with Amines

٣١Chapter 18 ٣١

CH2CH3 Zn(Hg)

HCl, H2O

CH2CH2CH3

H HCl, H2O

Zn(Hg)CH2 CH3

Homework: Complete the following equations

٣٢Chapter 18 ٣٢

Formation of Acetals

• Acetals can easily be converted into carbonyl compounds and alcohol in the presence of dilute acid (H3O+) (remember reaction is reversible) however, they are stable in the presence

of base.

٣٣Chapter 18 ٣٣

Cyclic Acetals

• Addition of a diol produces a cyclic acetal.• The reaction is reversible.• This reaction is used in synthesis to protect

carbonyls from reaction.

٣٤Chapter 18 ٣٤

Acetals as Protecting Groups

• Aldehydes are more reactive than ketones.

٣٥Chapter 18 ٣٥

Reaction and Deprotection

OO1) NaBH4

2) H3O+

• The acetal will not react with NaBH4, so only the ketone will get reduced.

• Hydrolysis conditions will protonate the alcohol and remove the acetal to restore the aldehyde.

٣٦Chapter 18

The Wittig Reaction

PPh3: triphenylphosphine

The Wittig reaction converts the carbonyl group into a new C═C double bond.

٣٧Chapter 18 ٣٧

Solved Problem

Show how you would use a Wittig reaction to synthesize 1-phenyl-1,3-butadiene.

٣٨Chapter 18 ٣٨

HomeworkShow how you would synthesize the following compound using Wittig reaction.1

Show how you would synthesize the following compound from starting materials containing no more than six carbon atoms.

(a) (b)

٣٩Chapter 18 ٣٩

Homework: choose the correct answer

(1) What is the IUPAC name for the following compound?

2,2,4-trimethylpentane(d)

2,4,4-trimethylpentane(c)

1,1,3,3-tetramethylbutane(b)

1,3-pentamethylpropane(a)

٤٠Chapter 18 ٤٠

Homework: choose the correct answer(2) When a double bond is formed between two atoms, one of the bonds is a sigma bond and the other is a pi bond. The pi bond is created by the overlap of...

s orbitals(d)

p orbitals(c)

sp3 hybrid orbitals(b)

sp2 hybrid orbitals(a)

٤١٤١

(3) For the following two pairs of molecules, identify which partner will have the higher boiling point.

(a) Pair 1: A; Pair 2: C

(b) Pair 1: B; Pair 2: D

(c) Pair 1: A; Pair 2: D

(d) Pair 1: B; Pair 2: C

٤٢Chapter 18 ٤٢

(4) What are the major alkenes formed in the following reaction?

٤٣Chapter 18 ٤٣

(5) Which product is obtained upon the reaction of (Z)-2-hexene with HBr?

(a) 2,3-Dibromohexane

(b) 2-Bromohexane

(c) 3-Bromohexane

(d) 2-Bromohexane and 3-bromohexane

٤٤Chapter 18 ٤٤

Homework: choose the correct answer(6) Which of the following compounds gives 3-methyl-2-butanol upon reduction with LiAlH4?

٤٥Chapter 18 ٤٥

(7) Which of the following sets of reaction conditions will give the product indicated?

(a) (CH3O)2Ag, heat

(b) CH3OH, HO- , separate water

(c) 1) CH3ONa, 2) MeI

(d) CH3OH, H+, separate water

٤٦Chapter 18 ٤٦

(8) What is the product of the following reaction?

٤٧Chapter 18 ٤٧

(9) When benzaldehyde reacts with HCN and catalytic amounts of NaCN in ethanol, the product is

CHEM-103: BASIC ORGANIC CHEMISTRY

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