Condensation RXNs and Elimination RXNs

ORGANIC CHEMISTRY REACTIONS

Condensation - Esterification• Condensation: Reactions in which water is eliminated •  Esterification: ester formed by combining an alcohol and carboxylic acid.

• Ex: Combine propanol with ethanoic acid:•

• Methanol Ethanoic acid Methyl ethanoate Water

• –OH group from alcohol combines with H from the carboxylic acid to form H2O

• New bond between O from carboxylic acid and C from alcohol

O C C

O H

H

HH C OH

H

H

C O

H

H

C C

O H

H

HH+

H2 O

H + H2 O

This bond is part of the alcohol

This bond is partof the acid

This is the new bond

Condensation - Esterification• Ester name is derived from the original alcohol and carboxylic acid• Alcohol is the first part of the name ( –anol becomes –yl)• Carboxylic acid is the second (-anoic acid becomes -anoate)

• Thus, methanol and ethanoic acid form methyl ethanoate

• Reaction is catalyzed by concentrated sulfuric acid• Typically sweet smelling

• Used as artificial flavoring agents• Have no –OH groups,

• NOT very soluble in water• Don’t have the ability to hydrogen bond • Unlike the original alcohol and carboxylic acid

Condensation - Polymerization• If the monomer contains two different functional groups, it can undergo a type of polymerization called condensation polymerization• A small molecule (often water) is released each time two

monomers come together.

•  Ex. Synthesis of proteins from amino acids:•  

• Amino acid – has an amine and a carboxylic acid• The two functional groups attached to a chiral carbon• Also bonded to a hydrogen atom and a unique side chain (represented by “R”)

• This side chain, R, is different for each amino acid (20 different amino acids, each with a different “R” group)

OHCC

O

N

H

H

H

Ramine group carboxylic acid group

side chain

Condensation - Polymerization• Amino acid – has an amine and a carboxylic acid

Condensation - Polymerization• When two amino acids come together

• “H” is lost from the amine group and the “OH” is lost from the carboxylic acid group

• This is a water molecule• New bond formed between the nitrogen in the amine group

and the carbon in the carboxylic acid group = peptide bond or peptide linkage

• Molecule is called a dipeptide• Can combine with other amino acids to eventually form a long

chain of amino acids called a protein.

OHCC

O

N

H

H

H

H

OHCC

O

N

H

H

H

CH3

CC

O

N

H

H

H

H

OHCC

O

N

H H

CH3

H2 O

peptide bondglycine alanine

Condensation - Polymerization• Optical isomerism in amino acids• Called the “L” form and one called the “D” form• Living organisms most proteins made of “L” form• Synthetic creation = a racemic mixture• Other examples of condensation polymers include nylon and polyester

• An example of the formation of a polyester:

Condensation vs Dehydration

• Condensation:• Possible small molecules lost are water, hydrogen chloride,

methanol, or acetic acid but most commonly in a biological reaction it is water.

• Often intermolecular (bonds 2 molecules together)

• Dehydration is a subset of Condensation (a type of)• Removes a water molecule from a larger molecule• Often needs a Bronstead acid catalyst

Elimination vs Substitution• Elimination reactions are similar to substitution reactions

• The differences:• In elimination, the halogen is removed along with a hydrogen atom• Creating an alkene

• In substitution, the halogen in a halogenoalkane is replaced with an –OH group• Creating an alcohol

• Difference of temperature• Substitution (60°C) vs elimination (100°C)

• Substitution occurs in a dilute solution of hydroxide ion• Elimination reactions occur in a higher concentration of hydroxide ions in ethanol

Elimination of halogen from haloalkane• Elimination of bromine from bromoethane:• A concentrated solution of hydroxide ion in ethanol is heated to approximately 100°C

• The hydroxide ion (strong base) is able to remove a hydrogen ion from the –OH group on the alcohol

• Creates molecule of water and an ion called the ethoxide ion (C2H5O-) (a strong base)

C C

H

H

H

H

H O H C C

H

H

H

H

H O:. .

. .:O: H

-

. .

. .:O: HH:

-

Elimination of halogen from haloalkane• Next, the ethoxide ion “attacks” a hydrogen atom on the bromoethane

molecule. Specifically, a hydrogen on the carbon adjacent to the bromine-containing carbon is attacked:

• ethoxide ion bromoethane ethanol ethene bromide ion

• The “curly arrows” are showing the movement of electron pairs• Ethoxide (strong base) removes a hydrogen ion from the bromoethane

molecule (the “attack”)• Electrons in the bond between this H and C remain with carbon (remember,

removed hydrogen ion)• Those two electrons move to form a double bond between the 2 C atoms• Increased electron density repels the electrons in the carbon-bromine bond

away• These electrons end up moving completely to the bromine atom to form a

bromide ion• This “eliminates” bromine from the bromoethane

C C

H

H

H

H

H BrC C

H

H

H

H

H O C C

H

H

H

H

C C

H

H

H

H

H O H Br:-

: + +