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10.2 PART 2NOTE: PARTS OF THIS POWERPOINT WERE FOUND ON THE WEB. IF YOU WANT THE SOURCES PLEASE LE ME
KNOW
Reactions containing
oxygen and
halogenalkanes
ALCOHOL REACTIONS
Alcohols contain an –OH group covalently bonded to a carbon atom.
However, this –OH group does not behave in the same way as the hydroxide ion OH– because NaOH is a base and CH3OH is not.
Alcohols, when dissolved in water, do not alter the pH of the water.
Although the hydrogen atom is connected to an oxygen atom, alcohols do not readily donate the proton (they are weaker acids than water).
ALCOHOL REACTIONS
Like the alkanes and alkenes, alcohols undergo completecombustion in a plentiful supply of oxygen gas, producing only
carbon dioxide and water as products.
When balancing an equation for the combustion of an alcohol it is important to remember that there is an oxygen atom in the alcohol, unlike alkanes and alkenes
The complete combustion of propanol is as follows:
ALCOHOL REACTIONS
The process of oxidation was defined as the loss of electrons.
In organic chemistry oxidation is easily recognized as the gain of oxygen or the loss of hydrogen from a compound.
The oxidation reactions of alcohols vary, depending upon the type of alcohol involved.
Primary, secondary and tertiary alcohols all give different reactions with strong oxidizing agents such as acidified potassium dichromate(VI) solution or acidified potassium manganate(VII) solution.
When these oxidation reactions are performed in a laboratory investigation, the change in colour of the oxidizing agent indicates that the reaction has proceeded.
Potassium dichromate, K2Cr2O7, changes colour from orange (Cr2O72-) to
green (Cr3+) during this reaction. If potassium manganate(VII), KMnO4, is used instead, it changes colour from purple to colourless.
ALCOHOL OXIDIZES TO CARBOXYLIC ACID
In the laboratory, when an aqueous solution of a primary alcohol such as ethanol is mixed with potassium dichromate (VI) and sulfuric acid, and the mixture heated under reflux, the alcohol is fully oxidized to a carboxylic acid.
During the process, the alcohol is initially oxidized to an aldehyde; however, by heating under reflux the aldehyde is further oxidized to a carboxylic acid.
When the reaction is ‘complete’, the condenser is turned around and the reaction mixture is distilled to collect an aqueous solution of the carboxylic acid.
If the aldehyde is the desired product during this reaction, then the reaction can be carried out at room temperature and the aldehyde can be distilled off from the mixture.
PRIMARY ALCOHOL OXIDIZES TO CARBOXYLIC ACID
PRIMARY ALCOHOL OXIDIZES TO CARBOXYLIC ACID
The oxidation reaction of the primary alcohol (e.g. ethanol) to a carboxylic acid may be represented simply by an equation in which the symbol [O] represents the oxygen supplied by the oxidizing agent:
8. OXIDATION OF PRIMARY ALCOHOL TO ALDEHYDESubtraction: Remove -H from alcohol group
Remove -H from alcohol carbon
HO
H
CH2
H2C
H3COH
C
H2C
H3CO
H
H
H + (O)
CH
H2C
H3CO
SECONDARY ALCOHOL OXIDATION
A secondary alcohol has the hydroxyl group on a carbon that is bonded to two other carbons. Propan-2-ol and butan-2-ol are examples of secondary alcohols.
SECONDARY ALCOHOL OXIDATION
When secondary alcohols are oxidized, ketones are formed.
This reaction is very similar to the one in which aldehydes are produced, but the placement of the hydroxyl group results in the production of a ketone rather than an aldehyde and, ultimately, a carboxylic acid.
SECONDARY ALCOHOL OXIDATION
The oxidation reaction of a secondary alcohol such as propan-2-ol to a ketone may be represented simply as:
9. OXIDATION OF SECONDARY ALCOHOL TO KETONESubtraction: Remove -H from alcohol group
Remove -H from alcohol carbon
HO
H
+ (O)
CH3
CHH3C
OH
CH3
CH3C
OH
H
CH3
CH3C
O
TERTIARY ALCOHOL OXIDATION
Tertiary alcohols, those with the hydroxyl group bonded to a carbon atom that is bonded to three other carbon atoms, are not easily oxidized.
An example of a tertiary alcohol is 2-methylpropan-2-ol.
REACTIONS OF ALCOHOLS
15
ESTERS
In an ester,
The H in the carboxyl group is replaced
with an alkyl group.
O
CH3 — C—O—CH3
ester group
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
ESTERS
Formed when an alcohol reacts with a carboxylic acid.
An acid catalyst used (usually sulphuric acid)
A condensation reaction
The condensation reaction between the hydroxyl group and the carboxylic acid known as esterification.
Reverse reaction = ester hydrolysis
This is why it is a condensation reaction because water is produced!
R
R’C
H2O
+
O H
H O
OR’C
R O
O
+
⇋
Definition of a condensation reaction =
two molecules reacting to form a larger
molecule with the elimination of a
small molecule such as water
R
R’C
H2O
+
O H
H O
OR’C
R O
O
+
⇌
FORWARD REACTION = condensation reaction,
the esterification of an alcohol using
acid catalyst under reflux.
REVERSE REACTION = ester hydrolysis, same
catalyst works for both forward & reverse
reactions.
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Esterification is
The reaction of a carboxylic acid and alcohol in the presence of an acid catalyst to produce an ester.
O
H+
CH3—C—OH + H—O—CH2—CH3
O
CH3—C—O—CH2—CH3 + H2O
ethyl acetate (an ester)
ESTERIFICATION
POLAR BONDS AND NUCLEOPHILESThe carbon–halogen bond in halogenoalkanes is polar because all
halogens are more electronegative than carbon.
The polar bond means that the carbon atom has a small positive
charge (δ+), which attracts substances with a lone pair of
electrons. These are nucleophiles, meaning ‘nucleus (positive
charge) loving’. Examples include:
δ+ δ- δ+ δ- δ+ δ- δ+ δ-
ammonia cyanide hydroxide
Nucleophiles (Nu-) attack the carbon of a
carbon–halogen (C–X) bond, because the
electron pair on the nucleophile is
attracted towards the small positive
charge on the carbon.
REACTION WITH NUCLEOPHILES
The electrons in the C–X bond are repelled
as the Nu- approaches the carbon atom.
δ+ δ-
The Nu- bonds to the carbon and the C–X
bond breaks. The two electrons move to the
halogen, forming a halide ion.
The halide is substituted, so this is a
nucleophilic substitution reaction.
HALOGENOALKANES NUCLEOPHILIC SUBSTITUTION REACTIONS
There are three that you should be familiar with:
Reagent Aqueous* sodium (or potassium)hydroxide
Product Alcohol
Nucleophile hydroxide ion (OH¯)
Equation
e.g. C2H5Br(l) + NaOH(aq) ——> C2H5OH(l) + NaBr(aq)
HALOGENOALKANES NUCLEOPHILIC SUBSTITUTION REACTIONS
There are three that you should be familiar with:
Reagent Aqueous, alcoholic potassium (or sodium) cyanide
Product Nitrile (cyanide)
Nucleophile cyanide ion (CN¯)
Equation
e.g. C2H5Br + KCN (aq/alc) ——> C2H5CN + KBr(aq)
HALOGENOALKANES NUCLEOPHILIC SUBSTITUTION REACTIONS
There are three that you should be familiar with:
Reagent Aqueous, alcoholic ammonia (in EXCESS)
Product Amine
Nucleophile Ammonia (NH3)
Equation
e.g. C2H5Br + 2NH3 (aq / alc) ——> C2H5NH2 + NH4Br
25
Electrophilic Aromatic Substitution
• The characteristic reaction of benzene is electrophilic aromatic
substitution—a hydrogen atom is replaced by an electrophile.
Background
26
• Benzene does not undergo addition reactions like other
unsaturated hydrocarbons, because addition would yield
a product that is not aromatic.
• Substitution of a hydrogen keeps the aromatic ring
intact.
• Electrophiles are things that love electrons because they
lack electrons themselves
27
• In halogenation, benzene reacts with Cl2 or Br2 in the
presence of a Lewis acid catalyst, such as FeCl3 or
FeBr3, to give the aryl halides chlorobenzene or
bromobenzene respectively.
• Analogous reactions with I2 and F2 are not synthetically
useful because I2 is too unreactive and F2 reacts too
violently.
Halogenation