Post on 05-Aug-2015
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Chapter 7
Alcohols, phenols, and Thiols
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Structure of Alcohols
Hydroxyl (-OH) functional groupOxygen is sp3 hybridized.
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Nomenclature of AlcoholsCompounds in which a hydrogen is replaced by an OH group.We distinguish three types:
C OHRR
R
• Tertiary alcohols
CH OHR
R• Secondary alcohols
CH2 OHR• Primary alcohols
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Nomenclature of AlcoholsCommon names are the name of alkyl group followed by the word “alcohol”
OH
cyclohexylalcohol
OH isopropyl alcohol
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Nomenclature of Alcohols
CH3CH2CH2CH2CHCH2OHCH2CH2CH3
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1 2 3
CNotice that numbering starts at alcohol group
2-propyl-1-hexanol
CH3CH2CH2CH2CHCH2OHCH2CH2CH3
CC
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More Examples
HO
H3C
Br
CH31
2 3
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3-bromo-2-pentanol
OH1
234
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2,3-dimethyl-1-cyclohexanol
2,3-dimethyl-cyclohexanol
OHH3C
CH3
65
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3-ethyl-5-methyl-1-cyclohexanol
3-ethyl-5-methyl-cyclohexanol
OH
H3C4
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4-methyl-cyclopent-2-en-1-ol
4-methyl-cyclopent-2-enol
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Naming PriorityAcidsEstersAldehydesKetonesAlcoholsAmines
AlkenesAlkynesAlkanesEthersHalides
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Hydroxy SubstituentWhen -OH is part of a higher priority class of compound, it is named as hydroxy.Example:
4-hydroxybutanoic acid
O
OHHO
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Nomenclature of PhenolsOH 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.OH is named as hydroxy.
OH
Cl
3-chlorophenolmeta-chlorophenol
OH
CHO4-hydroxybenzaldehyde
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Hydrogen Bonding in Alcohols and Phenols
CH3C
Clchloroethane
HH
MW 65bp 12.3
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Hydrogen Bonding in Alcohols and PhenolsUnusually high boiling points due to hydrogen bonding between molecules.A positively polarized ⎯OH hydrogen atom from one molecule is attracted to a lone pair of electrons on a negatively polarized oxygen atom of another moleculeThis produces a force that holds the two molecules togetherThese intermolecular attractions are present in solution but not in the gas phase, thus elevating the boiling point of the solution
O
R
H HO
H
R
O
R
O
R
H HO
H
R
O
R
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Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases.
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Acidity and Basicity ReviewedBrønsted–Lowry
Acid donates a proton Base accepts a proton
• Strong reacts to give weak• The weaker the base, the stronger is its conjugate acid• Stable bases are weak bases
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An Acid/Base Equilibrium
Ka: The acid dissociation constant
H2O + HA H3O+ + A-
[H3O+][A-]Ka = [H2O][HA]
pKa = -log Ka
The stronger the acid, the smaller is its pKa
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The Most Common Organic Acids Are Carboxylic Acids
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Lewis acid: accepting pair of electrons (AlCl3, BH3 and Mn+,like Zn2+, Mg2+, Na+, R3C+………)
Lewis base: donating pair of electrons ( H-, -OR, -NH2, R3C-, X-
H2O, NR3,
Lewis Acids and Bases
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The Acidity of Alcohols and PhenolsAlchols and Phenols are Weak Brønsted AcidsCan transfer a proton to water to a very small extentProduces H3O+ and an alkoxide ion, RO−, or a phenoxideion, ArO−
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pKa Values for Typical OH Compounds
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Relative Acidities of AlcoholsSimple alcohols are about as acidic as water.Alkyl groups make an alcohol a weaker acid.Electron-withdrawing groups make an alcohol a stronger acid by stabilizing the conjugate base (alkoxide)
Inductive Effects
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Generating Alkoxides from AlcoholsAlcohols are weak acids – requires a strong base to form an alkoxide such as NaH, or metal (K, Na) Alkoxides are bases used as reagents in organic chemistry
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Phenol Acidity
Phenols (pKa ~10) are much more acidic than alcohols (pKa~ 16) due to resonance stabilization of the phenoxide ion
Phenols react with NaOH solutions (but alcohols do not), forming soluble salts that are soluble in dilute aqueous
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Substituted Phenols
Can be more or less acidic than phenol itselfAn electron-withdrawing substituent makes a phenol more acidic by delocalizing the negative chargePhenols with an electron-donating substituent are less acidic because these substituents concentrate the charge
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Nitro-PhenolsPhenols with nitro groups at the ortho and para positions are much stronger acidsThe pKa of 2,4,6-trinitrophenol is 0.6, a very strong acid
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The Basicity of Alcohols and Phenols
Alcohols are weak Brønsted basesProtonated by strong acids to yield oxoniumions, ROH2
+
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Dehydration of Alcohols to AlkeneDehydration (elimination) of water is commonly obtained using sulfuric acid (H2SO4) as a catalyst.
H3C
CH3
OH H2SO4
ΔH3CHC CHCH3 + H2O
The acid is mandatory to convert the poor leaving group OH– into a good leaving group H2O.
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DehydrationFirst step is protonation of the hydroxyl group.
H3C
CH3
OH H2SO4
Δ
H3C
CH3
OH
H
H3C
CH3
OH
H - H2O H3C
CH3
Loss of water leads to a carbocation.
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DehydrationSecond, a base removes a proton β to the carbocation center.
H3C
CH3H Hα
β
OSO3H
H3CHC CHCH3
+H2SO4
Notice that this reaction is an E1 reaction.•Rate-determining step is the formation of the carbocation.
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DehydrationIn case we have a choice between several β-hydrogens, the most stable alkene usually predominates
CH3CHCH2CH3
CH3
OH
CHH3C
H3C
CH3
+ CH2H3C
H2C
CH3
H3PO4
Δ
84 % 16 %
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DehydrationAs a result of the E1 mechanism, the ease of dehydration follows the order:
OHR
RR
OHR
HR
OHH
RH
> >
RR
R
RH
R
H
RH
> >
That directly reflects the stability of the intermediate carbocations.
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DehydrationPrimary alcohols undergo dehydration by an E2 pathway.First, however, we generate the good leaving group.
OHH+
OH
H
H3C OH
H
HOSO3H
H3CCH
CH2
The subsequent steps, removal of water and deprotonation, take place simultaneously.
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The Reaction of Alcohols with Hydrogen Halides
HOHI
ΔI + H2O
cyclohexanol iodocyclohexane
OHHCl
ΔCl + H2O
1-propanol 1-chloropropane
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Primary, secondary, and tertiary alcohols all undergonucleophilic substitution reactions with HI, HBr, and HCl
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Secondary and tertiary alcohols undergo SN1 reactions with hydrogen halides
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Primary alcohols undergo SN2 reactions with hydrogen halides
ZnCl2 can be used to catalyze certain SN2 reactions
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Other Ways to Prepare Alkyl Halides from AlcoholsUsing thionyl chloride (SOCl2) and Phosphorous halide (PX3)
3° alcohols are converted by HCl or HBr at low temperature SOCl2 and PBr3 are used mainly for 1° and 2° alcohols since their reactions with HX are slow.
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A Comparison of Alcohols and Phenols
OH
HCl
OH
H
Cl-Cl
OH
HCl
OH
H
a phenyl cationvery unstablecan not formed
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Oxidation of AlcoholsCan be accomplished by inorganic reagents, such as KMnO4, CrO3, and Na2Cr2O7 or by more selective, expensive reagents
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Oxidation of Primary AlcoholsTo aldehyde: pyridinium chlorochromate (PCC, C5H6NCrO3Cl) in dichloromethaneOther reagents produce carboxylic acids
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Oxidation of Secondary Alcohols
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7.14 Reactions of PhenolsThe hydroxyl group is a strongly activating, making phenols substrates for electrophilichalogenation, nitration, sulfonation, and Friedel–Crafts reactionsReaction of a phenol with strong oxidizing agents yields a quinone
KMnO4Or
K2Cr2O7
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7.17 Thiols, Sulfur Analogs of Alcohols and Phenols
Thiols (RSH), are sulfur analogs of alcohols• Named with the suffix –thiol• (stronger acid than alcohol due to the size of
sulfur S larger than O )• SH group is called “mercaptan”
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Thiols: Formation and ReactionFrom alkyl halides by displacement with a sulfur nucleophile such as −SH • The alkylthiol product can undergo further reaction
with the alkyl halide to give a symmetrical sulfide, giving a poorer yield of the thiol
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SulfidesThiolates (RS−) are formed by the reaction of a thiol with a baseThiolates react with primary or secondary alkyl halide to give sulfides (RSR’)Thiolates are excellent nucleophiles and react with many electrophiles
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Oxidation of Thiols to DisulfidesReaction of an alkyl thiol (RSH) with bromine or iodine gives a disulfide (RSSR)The thiol is oxidized in the process and the halogen is reduced