Chapter 10 Alcohols 1 - San Diego Miramar Collegefaculty.sdmiramar.edu/choeger/Chapter 10 Alcohols...

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Chapter 10Structure and Synthesis

of Alcohols

Chemistry 231MiramarFall 2007

Chapter 10: Alcohols Slide 10-2

Structure of Alcohols

• Hydroxyl (-OH) functional group• Oxygen is sp3 hybridized.

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Classification• 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

(technically, not an alcohol!).

NOTE: An alcohol does NOT have an OH bound to an sp2carbon!


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, starting from the end closest to the -OH

group.• Number and name all substituents.• Unsaturated alcohols: -OH takes priority

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Naming Priority

• Acids• Esters and Amides• Aldehydes• Ketones• Alcohols• Amines

• Alkenes• Alkynes• Alkanes• Ethers• Halides


Hydroxy Substituent• When -OH is part of a higher priority class of compound, it is

named as hydroxy.• Example:

also known as GHB

O

OH

HO

4-hydroxybutanoic acid!

"

#

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Common Names• Alcohol can be named as alkyl alcohol.• Useful only for small alkyl groups.• Examples:

isobutyl alcoholIsopropyl alcoholEthyl alcoholTert-butyl alcohol


Naming Diols• Two numbers are needed to locate the two

-OH groups.• Use -diol as suffix instead of -ol.

hexane-1,6- diol

=>

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Glycols• 1, 2 diols (vicinal diols) are called glycols.• Common names for glycols use the name of the alkene from

which they were made.

ethane-1,2- diol

ethylene glycol

propane-1,2- diolpropylene glycol =>


Physical Properties• Unusually 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.

=>

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Boiling Points

=>


Solubility in Water

Solubility decreases as the size of the alkyl group increases.

=>

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Methanol• “Wood alcohol”• Industrial production from synthesis gas• Common industrial solvent• Fuel at Indianapolis 500

Fire can be extinguished with waterHigh octane ratingLow emissionsBut, lower energy content Invisible flame =>


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 ethanol, 100 mL methanol

=>

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Acidity of Alcohols• pKa range: 15.5-18.0 (water: 15.7)• Acidity decreases as alkyl group increases.• Halogens increase the acidity.• Phenol is 100 million times more acidic than cyclohexanol!

=>


Table of Ka Values

=>

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Formation of Alkoxide Ions

React methanol and ethanol with sodium metal or NaH (redoxreaction).

React less acidic alcohols with more reactive potassium or KH.

CH3CH2 OH + Na (s) CH3CH2 ONa + H2 (g)

(CH3)3C OH + K (s) (CH3)3C OK + H2 (g)


Formation of Phenoxide IonPhenol reacts with hydroxide ions to form phenoxide ions - no

redox is necessary.

O H+ OH

O

+ HOH

pKa = 10.0pKa = 15.7

=>

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Synthesis (Review)• Nucleophilic substitution of OH- on alkyl halide• Hydration of alkenes

water in acid solution (not very effective)oxymercuration - demercurationhydroboration - oxidation

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Glycols (Review)• Syn hydroxylation of alkenes

osmium tetroxide, hydrogen peroxide cold, dilute, basic potassium permanganate

• Anti hydroxylation of alkenesperoxyacids, hydrolysis

=>

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Organometallic Reagents• Carbon is bonded to a metal (Mg or Li).• Carbon is nucleophilic (partially negative).• It will attack a partially positive carbon.

C - XC = O

• A new carbon-carbon bond forms. =>


Grignard Reagents• Formula R-Mg-X (reacts like R:- +MgX)• Stabilized by anhydrous ether• Iodides most reactive• May be formed from any halide

primary secondary tertiaryvinyl aryl =>

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Some Grignard Reagents


Organolithium Reagents• Formula R-Li (reacts like R:- +Li)• Can be produced from alkyl, vinyl, or aryl halides, just like

Grignard reagents.• Ether not necessary, wide variety of solvents can be used.

=>

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Reaction with Carbonyl• R:- attacks the partially positive carbon in the carbonyl.• The intermediate is an alkoxide ion.• Addition of water or dilute acid protonates the alkoxide to

produce an alcohol.


Synthesis of 1° AlcoholsGrignard + formaldehyde yields a primary alcohol with one

additional carbon. Reaction with ethylene oxide yields aprimary alcohol with TWO additional carbons:

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Synthesis of 2º AlcoholsGrignard + aldehyde yields a secondary alcohol.


Synthesis of 3º AlcoholsGrignard + ketone yields a tertiary alcohol.

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Grignard Reactions with Acid Chlorides and Esters

• Use two moles of Grignard reagent.• The product is a tertiary alcohol with

two identical alkyl groups.• Reaction with one mole of Grignard reagent produces a ketone

intermediate, which reacts with the second mole of Grignardreagent.


Grignard + Acid Chloride (1)

• Grignard attacks the carbonyl.• Chloride ion leaves.• Ketone intermediate

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Grignard and Ester (1)• Grignard attacks the carbonyl.• Alkoxide ion leaves! ? !


Second step of reaction• Second mole of Grignard reacts with the ketone intermediate

to form an alkoxide ion.• Alkoxide ion is protonated with dilute acid.

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Grignard Reagent + Ethylene Oxide

• Epoxides are unusually reactive ethers.• Product is a 1º alcohol with 2 additional carbons.


Limitations of Grignard• No water or other acidic protons like

O-H, N-H, S-H, or -C—C-H. Grignard reagent is destroyed,becomes an alkane.

• No other electrophilic multiple bonds, like C=N, C≡N, S=O,or N=O.

=>

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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

=>


Sodium Borohydride

• Hydride ion, H-, attacks the carbonyl carbon, forming analkoxide 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.

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Lithium Aluminum Hydride

• Stronger reducing agent than sodium borohydride, butdangerous to work with.

• Converts esters and acids to 1º alcohols.


Comparison ofReducing Agents

• LiAlH4 is stronger.• LiAlH4 reduces more stable

compounds which are resistantto reduction. =>

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Catalytic Hydrogenation• Add H2 with Raney nickel catalyst.• Also reduces any C=C bonds.


Thiols (Mercaptans)• Sulfur analogues of alcohols, -SH.• Named by adding -thiol to alkane name.• The -SH group is called mercapto.• Complex with heavy metals: Hg, As, Au.• More acidic than alcohols, react with NaOH to form thiolate

ion.• Stinks! =>

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Thiol SynthesisUse a large excess of sodium hydrosulfide with unhindered alkyl

halide to prevent dialkylation to R-S-R.


Thiol Oxidation• Easily oxidized to disulfides, an important feature of protein

structure.

• Vigorous oxidation with KMnO4, HNO3, or NaOClproduces sulfonic acids.

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End of Chapter 10

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