CHEM1102 Lecture Notes 12
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Transcript of CHEM1102 Lecture Notes 12
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Chemistry 1102Charlie BondMCS Rm 4.16/[email protected]
What is Organic Chemistry?Organic Reactions I IIAlkanes (Ch 21)Conformational Analysis (Ch 21)Stereochemistry I II III (Ch 22)
S
Alkyl Halides I IIII (Ch 24)Alcohols and Ether I II (Ch 24)
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2Oxidation of alcoholsPrimary alcohols can be oxidised to aldehydes or carboxylic acids[O] = 1 oxidising equivalent, a generic way of describing an oxidant
Secondary alcohols can be oxidised to ketones:
[O][O]
[O]
Tertiary alcohols cannot be simply oxidised:
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3Oxidation of Alcohols Oxidation of a 1 alcohol gives an aldehyde or a
carboxylic acid, depending on the oxidizing agent and experimental conditions the most common oxidizing agent is chromic acid
Eg chromic acid oxidation of 1-octanol gives octanoic acid
CrO3 H2OH2SO4 H2CrO4+
ChromicacidChromium(VI)oxide
CH3(CH2)6CH2OHCrO3
H2SO4, H2OCH3(CH2)6CH
OCH3(CH2)6COH
O
Octanal(notisolated)
Octanoicacid1Octanol
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4Oxidation of Alcohols to oxidize a 1 alcohol to an aldehyde, use PCC
PCC oxidation of geraniol gives geranial
Tertiary alcohols are not oxidized by either of these reagents; they are resistant to oxidation
CrO3 HClN N
H
CrO3Cl-
Pyridiniumchlorochromate(PCC)
Pyridine
+ ++
OHPCC
CH2Cl2 H
O
Geraniol Geranial
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5Demonstrations
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6Ethers - Structure
The functional group of an ether is an oxygen atom bonded to two carbon atoms oxygen is sp3 hybridized with bond angles of
approximately 109.5 in dimethyl ether, the C-O-C bond angle is
110.3
H
H O
H
C H
H
H
C
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7Ethers - Nomenclature IUPAC
the longest carbon chain is the parent alkane name the -OR group as an alkoxy substituent
Common names: name the groups bonded to oxygen followed by the
word etherether
CH3
CH3CH3OCCH3
O Et2O
OH
OEt
Ethoxyethane(Diethylether)
2Methoxy2methylpropane(methyltertbutylether,MTBE)
trans2Ethoxycyclohexanol
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8Ethers - Physical Properties
Ethers are polar molecules each C-O bond is polar covalent however, only weak attractive forces exist
between ether molecules
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9Ethers - Physical Properties boiling points are lower than those of alcohols
CH3CH2OHCH3OCH3
CH3CH2CH2CH2CH2OHHOCH2CH2CH2CH2OHCH3CH2CH2CH2OCH3
CH3CH2CH2CH2OHCH3CH2OCH2CH3
CH3OCH2CH2OCH3 ethyleneglycoldimethylether
90 84 infinite
8g/100g3574diethylether
1butanol 74 117 7.4g/100g
slight7188butylmethyletherinfinite230901,4butanediol2.3g/100g138881pentanol
7.8g/100g2446dimethyletherinfinite7846ethanol
SolubilityinWater
BoilingPoint(C)
MolecularWeightNameStructuralFormula
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10
Ethers - Physical Properties ethers are hydrogen bond acceptors How many H-bonds can they accept?
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11
Reactions of Ethers Ethers resemble hydrocarbons in their resistance to
chemical reaction they do not react with strong oxidizing agents such as
chromic acid, H2CrO4 they are not affected by most acids and bases at
moderate temperatures Because of their good solvent properties and general
inertness to chemical reaction, ethers are excellent solvents in which to carry out organic reactions
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12
Williamson Synthesis of EthersThis method of synthesising ethers brings together two parts of alkyl halide and alcohol chemistry:
the first step is the formation of an alkoxide from an alcohol:
the second step involves nucleophilic substitution on an alky halide where the alkoxide acts as the nucleophile:
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13
Thiols - Structure The functional group of a thiol is an -SH
(sulfhydryl) group bonded to a tetrahedral carbon
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14
Thiols - Nomenclature IUPAC names:
the parent chain is the longest chain containing the -SH group
add -thiol thiol to the name of the parent chain Common names:
name the alkyl group bonded to sulfur followed by the word mercaptanmercaptan
alternatively, indicate the -SH by the prefix mercaptomercapto
Ethanethiol(Ethylmercaptan)
2Methyl1propanethiol(Isobutylmercaptan)
2Mercaptoethanol
SH SH HS OH
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15
Thiols - Physical Properties
Low-molecular-weight thiols have a STENCHSTENCH
CH3CH=CHCH2SH CH3CHCH2CH2SHCH3
3Methyl1butanethiol(Isobutylmercaptan)
2Butene1thiolPresentinthe
scentofskunks:
CH3-C-SHCH3
CH3CH3-CH-CH3
SHNaturalgasodorants:
2Methyl2propanethiol(tertButylmercaptan)
2Propanethiol(Isopropylmercaptan)
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Thiols - Physical Properties
The difference in electronegativity between S and H is 2.5 - 2.1 = 0.4
Because of their low polarity, thiols show little association by hydrogen bonding have lower boiling points and are less soluble
in water than alcohols of comparable MW
1177865
1butanolethanolmethanol
98356
1butanethiolethanethiolmethanethiol
AlcoholBoilingPoint
(C)ThiolBoilingPoint
(C)
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17
Acidity of Thiols
Thiols are stronger acids than alcohols
Thiols react with strong bases to form salts
CH3CH2SH
CH3CH2OH
H2O
H2O
CH3CH2S-
CH3CH2O- H3O+
H3O+ pKa=8.5
pKa=15.9
+
++
+
CH3CH2SH Na+OH- CH3CH2S
-Na+ H2O+ +
Strongeracid
Strongerbase
Weakerbase
Weakeracid
pKa8.5 pKa15.7
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18
Oxidation of Thiols
thiols are oxidized by a variety of oxidizing agents, including O2, to disulfides
disulfides, in turn, are easily reduced to thiols by several reagents
this easy interconversion between thiols and disulfides is very important in protein chemistry
2HOCH2CH2SH HOCH2CH2S-SCH2CH2OHAdisulfide
oxidation
reductionAthiol
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