Lecture 5 Alkohol

5/23/2018 Lecture 5 Alkohol

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C 2004 Barry Linkletter, UPEI

LECTURE 5


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2

Introduction

Nomenclature Properties :-

Physical

Chemical

Reactions:- Hydration of alkene

Reduction of carbonyl

Carbonyl + Grignard reagents Dehydration

Conversion to R-X

Oxidation


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Alcohols contain an OH group connected to a a saturated C(sp3)

They are important solvents and synthesis intermediates

Methanol, CH3OH, called methyl alcohol, is a common solvent,

a fuel additive, produced in large quantities

Ethanol, CH3CH2OH, called ethyl alcohol, is a solvent, fuel,

beverage

Phenols contain an OH group connected to a carbon in a

benzene ring

Phenol, C6H5OH (phenyl

alcohol) has diverse uses - it

gives its name to the general

class of compounds


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Some Common Alcohols


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Some Important Alcohols


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Some Important Phenols


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Step 1. Name the longest chain to which the hydroxyl (OH)group is attached. The name for this chain is obtained by

dropping the final -e fromthe name of the hydrocarbon

parent name and adding the ending -ol.

Step 2. Number the longest chain to give the lowest possiblenumber to the carbon bearing the hydroxyl group.

Step 3. Locate the position of the hydroxyl group by the number of

the C to which it is attached.

Step 4. Locate and name any other substituents.

Step 5. Combine the name and location for other groups, the

hydroxyl group location, and the longest chain into the

final name.


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Give the IUPAC name for each of the following compounds:

2,5-heptadiol3-methyl-1-butanol

4-ethyl-3-hexanol

3-bromo-2-pentanol

a) b)

c) d)

C


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Give the IUPAC name for each of the following compounds:

4-ethyl-2-heptanol

2-methyl-1-butanol

3-methyl-3-hexanol

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Classification of alcohol

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Hydrogen Bonding The structure around O of the alcohol or phenol is similar to

that in water, sp3hybridized

The oxygen-hydrogen bond is an especially polar bond

because oxygen is much more electronegative than hydrogenis.

The OH bond is therefore a polar bond, and any molecule

which contains an OH bond (like an alcohol) is a polar

molecule.

C 2004 Barry Linkletter UPEI


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Hydrogen Bonding A positively polarized hydrogen atom from one molecule is

attracted to a lone pair of electrons on a negatively polarized

oxygen atom of another molecule

This produces a force that holds the two molecules together

These intermolecular

attractions are present

in solution but not inthe gas phase, thus

elevating the boiling

point of the solution



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Boiling Points

Because alcohols hydrogen bond to each other, they have much

higher boiling points than similar alkanes and alkyl halides.

The boiling point of alcohols increases as the molecules become

larger.



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Solubility of alcohols

The general rule in solubility is like dissolves like.

Since the OH group makes alcohols polar, they will mix with

polar solvents like water as long as the carbon chain is

fairly short.The longer the carbon chain, the less soluble the alcohol is.



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Solubility of alcohols

Alcohols with higher molar masses are less soluble in water

because the increasing size of the nonpolar chain disrupts

the hydrogen bonding network.

There is a solubility limit on the carbon chain length foralcohols.

In general, a singleOH group will make alcohols containing

three to four carbon atoms soluble in water.

As the number of carbons in the alcohol increases, thesolubility decreases.



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Alcohols are weak Brnsted bases

Protonated by strong acids to yield oxonium ions, ROH2+

Alcohol and Phenol Acid-Base Properties



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Alcohols and phenols also are weak Brnsted acids

Can transfer a proton to water to a very small extent

Produces H3O+and an alkoxide ion, RO, or a phenoxide

ion, ArO

Alcohol and Phenol Acid-Base Properties



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y ,

Brnsted acidity measurements

The acidity constant, Ka, measure the extent to which a

Brnsted acid transfers a proton to water

and pKa= log Ka

Relative acidities are more conveniently presented on a

logarithmic scale, pKa, which is directly proportional to the

free energy of the equilibrium

Differences in pKacorrespond to differences in free energy



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Generating Alkoxides from Alcohols Alcohols are weak acidsrequires a strong base to form

an alkoxide such as NaH, sodium amide (NaNH2), and

Grignard reagents (RMgX)

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

A phenolic component can be separated from an organic

solution by extraction into basic aqueous solution and is

isolated after acid is added to the solution



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Alcohols are derived from many types of compounds The alcohol hydroxyl can be converted to many other

functional groups

This makes alcohols useful in synthesis



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REVIEW: PREPARATION OF ALCOHOLS BY

REGIOSPECIFIC HYDRATION OF ALKENES

Hydroboration/oxidation: syn, non-Markovnikov hydration

Oxymercuration/reduction: Markovnikov hydration



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Reduction of a carbonyl compound in general gives an

alcohol

Note that organic reduction reactions add the equivalent

of H2to a molecule



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Aldehydes gives primary alcohols

Ketones gives secondary alcohols



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Sodium borohydride, NaBH4is not sensitive to moisture and it

does not reduce other common functional groups

Lithium aluminum hydride (LiAlH4) is more powerful, less

specific, and very reactive with water

Both add the equivalent of H



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The reagent adds the equivalent of hydride to the carbon ofC=O and polarizes the group as well



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Carboxylic acids and esters are reduced to give primary alcohols LiAlH4is used because NaBH4is not effective



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Alkyl, aryl, and vinylic halides react with magnesium in ether or

tetrahydrofuran to generate Grignard reagents, RMgX

Grignard reagents react with carbonyl compounds to yield

alcohols



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Grignard reagents act as nucleophilic carbon anions

(carbanions, :R) in adding to a carbonyl group The intermediate alkoxide is then protonated to produce the

alcohol



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Two general classes of reaction

At the carbon of the CO bond

At the proton of the OH bond



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DEHYDRATION OF ALCOHOLS TO YIELD

ALKENES

Alcohols undergo dehydration (loss of water) when they are

heated in the presence of a strong acid catalyst.

TheOH and anH are removed from adjacent carbon atoms

to yield an alkene.

Example

The dehydration of alcohols is the reverse of the hydration of

alkenes



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ALKENES

If there is more than one possible product of a dehydration

reaction, the major product can be predicted from Zaitsevs

Rule:

Zaitsevs Rulewhen an alkene is produced in an elimination

reaction, the major product is the one with the more highlysubstituted double bond.



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Tertiary alcohols are readily dehydrated with acid

Secondary alcohols require severe conditions (75% H2SO4,

100C) - sensitive molecules don't survive

Primary alcohols require very harsh conditionsimpractical


ALKENES



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The first step in this reaction is protonation of the hydroxyl

group, converting it into a good leaving group (H2O) .

Then, a C-O bond is converse into alkyl halides when treated

with hydrochloric/bromic acids (HCl or HBr).

Tertiary alcohols then ionize to the 3 carbocation which

undergoes an SN1 reaction with X-.

Primary alcohols react by an SN2 displacement of water from

the substrate by Cl-.

Secondary alcohols may react by either an SN1 or SN2

mechanism depending on the structure of the 2 alcohol.



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Tertiary alcohols react reasonably rapidly with concentrated

HCl or HBr at low temperature

A tertiary alcohol reacts if it is shaken with concentrated

hydrochloric acid at room temperature. A tertiaryhalogenoalkane (haloalkane or alkyl halide) is formed.



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1and 2alcohols are resistant to aciduse SOCl2(Thionyl

chloride) or PBr3(Phosphorus tribromide) by an SN2

mechanism



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Can be accomplished by inorganic reagents, such asKMnO4, CrO3, and Na2Cr2O7or by more selective, expensivereagents



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Alcohol forms a chromate ester followed by elimination withelectron transfer to give ketone.

The mechanism was determined by observing the effects of

isotopes on rates



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Primary alcohols are oxidized first to aldehydes, but thealdehydes are then usually oxidized into carboxylic acids

(using pyridinium chlorochromate (PCC, C5H6NCrO3Cl) in

dichloromethane).



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Secondary alcohols are oxidized to ketones,which cannot beoxidized any further:

Effective with

inexpensive reagents

such as Na2Cr2O7inacetic acid (PCC is

used for sensitive

alcohols at lower

temperatures).



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Lecture 5 Alkohol