Chapter 5 Alcohols Thiols Ethers
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Transcript of Chapter 5 Alcohols Thiols Ethers
Chapter 5Alcohols Thiols
Ethers
Chapter 10 2
Structure of Water and Methanol
• Oxygen is sp3 hybridized and tetrahedral.• The H—O—H angle in water is 104.5°. • The C—O—H angle in methyl alcohol is 108.9°.
Chapter 10 3
Classification of Alcohols
• Primary: carbon with —OH is bonded to one other carbon.
• Secondary: carbon with —OH is bonded to two other carbons.
• Tertiary: carbon with —OH is bonded to three other carbons.
• Aromatic (phenol): —OH is bonded to a benzene ring.
Chapter 10 4
Examples of Classifications
CH3 C
CH3
CH3
OH*
CH3 CH
OH
CH2CH3*
CH3 CH
CH3
CH2OH*
Chapter 10 5
Examples of Classifications
CH3 C
CH3
CH3
OH*
CH3 CH
OH
CH2CH3*
CH3 CH
CH3
CH2OH*
Primary alcohol
Chapter 10 6
Examples of Classifications
CH3 C
CH3
CH3
OH*
CH3 CH
OH
CH2CH3*
CH3 CH
CH3
CH2OH*
Primary alcohol Secondary alcohol
Chapter 10 7
Examples of Classifications
CH3 C
CH3
CH3
OH*
CH3 CH
OH
CH2CH3*
CH3 CH
CH3
CH2OH*
Primary alcohol Secondary alcohol
Tertiary alcohol
Chapter 10 8
IUPAC Nomenclature
• Find the longest carbon chain containing the carbon with the —OH group.
• Drop the -e from the alkane name, add -ol.• Number the chain giving the —OH group the lowest
number possible.• Number and name all substituents and write them in
alphabetical order.
Chapter 10 9
Examples of Nomenclature
2-methyl-1-propanol2-methylpropan-1-ol
2-methyl-2-propanol2-methylpropan-2-ol
2-butanolbutan-2-ol
CH3 C
CH3
CH3
OH
CH3 CH
CH3
CH2OH CH3 CH
OH
CH2CH33 2 1 1 2 3 4
2 1
Chapter 10 10
Alkenols (Enols)
• Hydroxyl group takes precedence. Assign the carbon with the —OH the lowest number.
• End the name in –ol, but also specify that there is a double bond by using the ending –ene before -ol
4-penten-2-ol pent-4-ene-2-ol
CH2 CHCH2CHCH3
OH
5 4 3 2 1
Chapter 10 11
Naming Priority
1. Acids2. Esters3. Aldehydes4. Ketones5. Alcohols6. Amines 7. Alkenes8. Alkynes9. Alkanes10. Ethers11. Halides
Highest ranking
Lowest ranking
Chapter 10 12
Hydroxy Substituent
• When —OH is part of a higher priority class of compound, it is named as hydroxy.
4-hydroxybutanoic acidalso known as -hydroxybutyric acid (GHB)
CH2CH2CH2COOH
OHcarboxylic acid
4 3 2 1
Chapter 10 13
Common Names
• Alcohol can be named as alkyl alcohol.• Useful only for small alkyl groups.
isobutyl alcohol sec-butyl alcohol
CH3 CH
CH3
CH2OH CH3 CH
OH
CH2CH3
Chapter 10 14
Naming Diols
• Two numbers are needed to locate the two —OH groups.
• Use -diol as suffix instead of -ol.
hexane-1,6- diol
1 2 3 4 5 6
OHOH
Chapter 10 15
Glycols• 1, 2-diols (vicinal diols) are called glycols.• Common names for glycols use the name of the alkene
from which they were made.
OHOH
ethane-1,2- diolethylene glycol
propane-1,2- diolpropylene glycol
OHOH
Chapter 10 16
Phenol Nomenclature• —OH group is assumed to be on carbon 1.• For common names of disubstituted phenols, use
ortho- for 1,2; meta- for 1,3; and para- for 1,4.• Methyl phenols are cresols.
3-chlorophenol(meta-chlorophenol)
4-methylphenol(para-cresol)
OH
Cl
OH
H3C
Chapter 10 17
Give the systematic (IUPAC) name for the following alcohol.
The longest chain contains six carbon atoms, but it does not contain the carbon bonded to the hydroxyl group. The longest chain containing the carbon bonded to the —OH group is the one outlined by the green box, containing five carbon atoms. This chain is numbered from right to left in order to give the hydroxyl-bearing carbon atom the lowest possible number.
The correct name for this compound is 3-(iodomethyl)-2-isopropylpentan-1-ol.
Solved Problem 1
Solution
Chapter 10 18
Physical Properties
• Alcohols have high boiling points due to hydrogen bonding between molecules.
• Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases.
Chapter 10 19
Boiling Points of alcohols
• Alcohols have higher boiling points than ethers and alkanes because alcohols can form hydrogen bonds.
• The stronger interaction between alcohol molecules will require more energy to break them resulting in a higher boiling point.
Chapter 10 20
Solubility in Water
Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases.
Chapter 10 21
Methanol• “Wood alcohol”• Industrial production from synthesis gas• Common industrial solvent• Toxic Dose: 100 mL methanol• Used as fuel at Indianapolis 500
– Fire can be extinguished with water– High octane rating– Low emissions– Lower energy content– Invisible flame
Chapter 10 22
Ethanol
• Fermentation of sugar and starches in grains• 12–15% alcohol, then yeast cells die• Distillation produces “hard” liquors• Azeotrope: 95% ethanol, constant boiling• Denatured alcohol used as solvent• Gasahol: 10% ethanol in gasoline• Toxic dose: 200 mL
Chapter 10 23
Acidity of Alcohols
• pKa range: 15.5–18.0 (water: 15.7)
• Acidity decreases as the number of carbons increase.
• Halogens and other electron withdrawing groups increase the acidity.
• Phenol is 100 million times more acidic than cyclohexanol!
Chapter 10 24
Table of Ka Values
Chapter 10 25
Formation of Alkoxide Ions
• Ethanol reacts with sodium metal to form sodium ethoxide (NaOCH2CH3), a strong base commonly used for elimination reactions.
• More hindered alcohols like 2-propanol or tert-butanol react faster with potassium than with sodium.
Chapter 10 26
Formation of Phenoxide Ion
The aromatic alcohol phenol is more acidic than aliphatic alcohols due to the ability of aromatic rings to delocalize the negative charge of the oxygen within the carbons of the ring.
Chapter 10 27
Charge Delocalization on the Phenoxide Ion
• The negative charge of the oxygen can be delocalized over four atoms of the phenoxide ion.
• There are three other resonance structures that can localize the charge in three different carbons of the ring.
• The true structure is a hybrid between the four resonance forms.
Chapter 10 28
Synthesis of Alcohols (Review)
• Alcohols can be synthesized by nucleophilic substitution of alkyl halide.
• Hydration of alkenes also produce alcohols:
Chapter 10 29
Synthesis of Vicinal Diols
Vicinal diols can be synthesized by two different methods:
• Syn hydroxylation of alkenes– Cold, dilute, basic potassium permanganate
Chapter 10 30
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
Chapter 10 31
Sodium Borohydride
• NaBH4 is a source of hydrides (H-)• Hydride attacks the carbonyl carbon,
forming an alkoxide ion.• Then the alkoxide ion is protonated by
dilute acid.• Only reacts with carbonyl of aldehyde or
ketone, not with carbonyls of esters or carboxylic acids.
Chapter 10 32
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 1º
alcohols.
Chapter 10 33
Reduction with LiAlH4
O
OCH3 OH1. LAH2. H3O+
H H
• The LiAlH4 (or LAH) will add two hydrides to the ester to form the primary alkyl halide.
• The mechanism is similar to the attack of Grignards on esters.
Chapter 10 34
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.
Chapter 10 35
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 are more selective so they are used more frequently for carbonyl reductions.
Chapter 10 36
Thiols (Mercaptans)
• Sulfur analogues of alcohols are called thiols.• The —SH group is called a mercapto group.• Named by adding the suffix -thiol to the
alkane name.• They are commonly made by a substitution. • Primary alkyl halides work better.
Chapter 10 37
Synthesis of Thiols
• The thiolate will attack the carbon displacing the halide.
• This is a substitution reaction• methyl halides will react faster than primary alkyl
halides.• To prevent dialylation use a large excess of sodium
hydrosulfide with the alkyl halide.
Chapter 11 38
Alcohol Reactions
• Dehydration to alkene• Oxidation to aldehyde, ketone• Substitution to form alkyl halide• Reduction to alkane• Esterification• Williamson synthesis of ether
Chapter 11 39
Summary Table
Chapter 11 40
Oxidation States
• Easy for inorganic salts (reduced, organic oxidized)– CrO4
2- reduced to Cr2O3
– KMnO4 reduced to MnO2
• Oxidation: loss of H2, gain of O, O2, or X2
• Reduction: gain of H2 or H-, loss of O, O2, or X2
• Neither: gain or loss of H+, H2O, HX
Chapter 11 41
Oxidation States
• Easy for inorganic salts (reduced, organic oxidized)– CrO4
2- reduced to Cr2O3
– KMnO4 reduced to MnO2
• Oxidation: loss of H2, gain of O, O2, or X2
• Reduction: gain of H2 or H-, loss of O, O2, or X2
• Neither: gain or loss of H+, H2O, HX
Chapter 11 42
1º, 2º, 3º Carbons
Chapter 11 43
Oxidation of 2° Alcohols• 2° alcohol becomes a ketone• Reagent is Na2Cr2O7/H2SO4 = H2CrO4
• Active reagent probably H2CrO4
• Color change: orange to greenish-blue
CH3CHCH2CH3
OHNa2Cr2O7 / H2SO4
CH3CCH2CH3
O
=>
Chapter 11 44
Oxidation of 1° Alcohols• 1° alcohol to aldehyde to carboxylic acid• Difficult to stop at aldehyde• Use pyridinium chlorochromate (PCC) to
limit the oxidation.• PCC can also be used to oxidize 2° alcohols
to ketones.
CH3CH2CH2CH2
OH N H CrO3Cl
CH3CH2CH2CH
O
Chapter 11 45
3° Alcohols Don’t Oxidize• Cannot lose 2 H’s• Basis for chromic acid test
Chapter 11 46
Alcohol as a Nucleophile
• ROH is weak nucleophile• RO- is strong nucleophile• New O-C bond forms, O-H bond breaks.
CO
H
R X
Chapter 11 47
Alcohol as an Electrophile• OH- is not a good leaving group
unless it is protonated, but most nucleophiles are strong bases which would remove H+.
• Convert to tosylate (good leaving group) to react with strong nucleophile (base).
CO
H
+
C-Nuc bond forms, C-O bond breaks
Chapter 11 48
Reduction of Alcohols
• Dehydrate with conc. H2SO4, then add H2
CH3CHCH3
OHH2SO4
CH2 CHCH3H2
PtCH3CH2CH3
alcohol alkene alkane
Chapter 11 49
Reaction with HBr• -OH of alcohol is protonated• -OH2
+ is good leaving group
• 3° and 2° alcohols react with Br- via SN1
• 1° alcohols react via SN2
H3O+
Br-
R O H R O H
H
R Br
Chapter 11 50
Reaction with HCl• Chloride is a weaker nucleophile than
bromide.• Add ZnCl2, which bonds strongly with
-OH, to promote the reaction.• The chloride product is insoluble.• Lucas test: ZnCl2 in conc. HCl
– 1° alcohols react slowly or not at all.– 2 alcohols react in 1-5 minutes.– 3 alcohols react in less than 1 minute.
Chapter 11 51
Limitations of HX Reactions
• Poor yields of 1° and 2° chlorides• May get alkene instead of alkyl halide
Chapter 11 52
Reactions with Phosphorus Halides• Good yields with 1° and 2° alcohols• PCl3 for alkyl chloride (but SOCl2 better)• PBr3 for alkyl bromide
Chapter 11 53
Dehydration Reactions• Conc. H2SO4 (or H3PO4) produces alkene
• Carbocation intermediate• Zaitsev product• Bimolecular dehydration produces ether• Low temp, 140°C and below, favors ether• High temp, 180°C and above, favors alkene
Ethers
O
H H
O
H R
O
R R
Hydrogen OxideAka Water
Alcohol Ether
• Ethers contain an sp3 hybridized oxygen atom
• Ethers do not hydrogen bond between each other, but will hydrogen bond with water and alcohols.
• Ethers are polar and water soluble
Ether NomenclatureCommon System
Give alkyl names to the carbon groups (alkyl groups) bonded to the oxygen, followed by ether.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Ether NomenclatureCommon System
Give alkyl names to the carbon groups (alkyl groups) bonded to the oxygen, followed by ether.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Dimethyl ether
Ether NomenclatureCommon System
Give alkyl names to the carbon groups (alkyl groups) bonded to the oxygen, followed by ether.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Dimethyl ether Ethylmethyl ether
Ether NomenclatureCommon System
Give alkyl names to the carbon groups (alkyl groups) bonded to the oxygen, followed by ether.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Dimethyl ether Ethylmethyl ether
Isopropylmethyl ether
Ether NomenclatureCommon System
Give alkyl names to the carbon groups (alkyl groups) bonded to the oxygen, followed by ether.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Dimethyl ether Ethylmethyl ether
Isopropylmethyl ether Sec-butylcyclopropyl ether
Ether Nomenclature
IUPAC System
Name the longest chain of carbons in the normal fashion. The oxygen containing group is named by giving the carbon portion the Latin root followed by oxy.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Methoxymethane
Ether Nomenclature
IUPAC System
Name the longest chain of carbons in the normal fashion. The oxygen containing group is named by giving the carbon portion the Latin root followed by oxy.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Methoxymethane Methoxyethane
Ether Nomenclature
IUPAC System
Name the longest chain of carbons in the normal fashion. The oxygen containing group is named by giving the carbon portion the Latin root followed by oxy.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Methoxymethane Methoxyethane2-Methoxypropane
Ether Nomenclature
IUPAC System
Name the longest chain of carbons in the normal fashion. The oxygen containing group is named by giving the carbon portion the Latin root followed by oxy.
Examples
O
H3C CH3
O
H3C CH2CH3
O O
Methoxymethane Methoxyethane2-Methoxypropane 2-Cyclopropoxybutane
Ether Formation• Primary alcohols can dehydrate to ethers
• This reaction occurs at lower temperature than the competing dehydration to an alkene•This method generally does not work with secondary or tertiary alcohols because elimination
competes stronglyThe mechanism is an SN2 reaction:
Williamson Ether Synthesis• Good for unsymmetrical ethers
Chapter 11 66
Dehydration Mechanisms
CH3CHCH3
OHH2SO4
alcoholCH3CHCH3
OH
H
CH3CHCH3
CH2 CHCH3H2O
CH3OH
H3O+
CH3OH CH3 OH2 CH3 O
H
CH3
H2OCH3OCH3
Chapter 11 67
Esterification• Fischer: alcohol + carboxylic acid• Nitrate esters• Phosphate esters
Chapter 11 68
Fischer Esterification
• Acid + Alcohol yields Ester + Water• Sulfuric acid is a catalyst.• Each step is reversible.
CH3 C OH
O
+ CH2CH2CHCH3
CH3
OHH
+
CH3C
O
OCH2CH2CHCH3
CH3
+ HOH
Chapter 11 69
Sulfate EstersAlcohol + Sulfuric Acid
+HO S
O
O
OH H O CH2CH3H
+
OCH2CH3
O
O
SHO
CH3CH2O H + OCH2CH3
O
O
SHOH
+
CH3CH2O S
O
O
OCH2CH3
=>
Chapter 11 70
Nitrate Esters
+ H O CH2CH3H
+
N OH
O
OOCH2CH3N
O
O
Chapter 11 71
Phosphate Esters
=>
Chapter 11 72
Phosphate Esters in DNA
=>
OCH2
HH
H
base
OP
O
O O
OCH2
HH
H
base
OP
O
O O
OCH2
HH
H
base
OP
O
O O
O
OCH2
HH
H
base
OP
O
O O
Chapter 11 73
End of Chapter 5