Alcohols Ethers

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The Syrian International Private University For Science And Technology   College of pharmacy

Organic Chemistry Second year students

:: Alcohols and Ethers ::

Ass.Professor Dr.Musasthanna D.Saud Reference: Organic Chemistry by Morrison and Boyed

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A Alcohols s Stru ucture of Alcohols : Alcoohols are orrganic comppounds of thhe general formula f R-O OH, where R is any alkkyl or substiituted alkyl grouup. R groupp may be priimary, seconndary, or teertiary; it maay be open chain or cycclic. It may contain othher funcctional grouup as well. For F examplee:

2-propen-1-ol Allyl alcoholl

Cyclohex xanol

2 2-chloroethano ol Ethyylene chlorohy ydrin

1,2,3-propaanetriol Glycerrol

2-Methyy-2-propanol tert-Bu utyl alcohol

Benzzyl alcohol

All alcohols coontain the hyydroxyl (-O OH) group, which w is thee functional that determ mine the prop perties charracteristic of o this familyy. The diffeerence in thee R groups will affect the t rate of w which the allcohol undergoes certaain reactionns. It also may affect the kind of reeaction. Phennols also coontain hydrooxyl group, but it is attached direcctly to the arromatic ringg and so, theey differ so marrkedly form alcohols inn their physiical and cheemical properties and tyype of reacttions they undergo. u Alcoohols couldd classifies as a primary, secondary, or tertiary according a too the kind oof carbon that bears thee -OH H group.

6.4 Nomencllature of alcohols: a Aasaslso the coommon nam mes are usedd for simple alcohols which w consisst of the nam me of the alk kyl group folloowed by thee word alcohol. For exaample:

E Ethylalcohol

(1o)

Isoproppyl alcohol

(22o)

Isobutyl allcohol

(1o)

tert-buty yl alcohol

(3o)

Thee most versaatile system is, of coursse, the IUPA AC. The rules are: 1- Choosinng the longeest continuoous carbon chain c that contains the –OH groupp. 2- Indicatee by a nasum mber the poosition of thee –OH grou up in the preesent chain, generally using u the low west assaposssible number of –OH. 3- Indicatee by numberr the positioons of otherr groups attaached to thee parent chaain.

M Methanol

1 1-Propanol

2,2-dimethyll-1-propanol

1-bromo--2-propanol

1,2 2-Ethanediol

2-methyl-1-butanol

6.5 Physical properties p s of Alcoh hols: Struucturally, ann alcohol is a compositee of an alkaane and wateer: R-H H H-OH R- OHH An alkkane An alccohol Water It coontains an alkane a like alkyl a group and water like l hydroxy yl group. Thhe –OH grooup gives th he alcohol itts charracteristic physical p properties, andd the alkyl group g depen nding upon its i size and shape, mod difies this propperties. Hyddroxyl groupp (-OH) is polar-it p conttains hydroggen bonded d to the highhly electroneegative elem ment oxygenn. Throough the hyydroxyl grouup, an alcohhol is capabble of hydrog gen bondingg:

Beccause of hyddrogen bondding, alcohools have higgher boiling points thann compoundds of similarr molecular weigght (aldehyde, ketons and a ethers). Thee solubility of o alcohols in water is also a due to the polarity y of the hydrroxyl groupp and to the ability to foorm hydrrogen bondds with wateer moleculess. But the efffect of the –OH groupp decreases w with increasse the size of o the alkyl a groupp (the numbeer of carbonn atoms), i.ee., the non-p polar portion of the alcohol molecu ule. t size of the t alkyl grooup (non-poolar portion n) is associatted with deccrease the solubility in So, increasing the wateer (aqueouss solubility). Induustrial sourcce of Alcohols: a- By hydrration of alkkenes obtainned from thee cracking of o petroleum m b- By the oxo o process from alkennes, carbon monoxide m and a hydrogeen. c- By ferm mentation off carbohydraate.

Alcohols as acids and bases: Like water, alcohols are weak acids and weak bases. The oxygen with its unshared electrons pairs, that makes as alcohol basic. R - OH + H 2 SO 4

ROH 2 + HSO4 +

In alcohols hydrogen is bonded to the very electronegative element oxygen. The polarity of the –OH bond facilitate the breaking of –OH bond. 1 ROH + Na RONa + H2 2 An alcohol (A weak acid)

RO - Na + + HOH Strong base strong acid

A sodium (alkoxide, a strong base)

NaOH + RO - H weaker base

weaker acid

Like water and ammonia, alcohols are stronger acids than alkanes, and readily displace them from their salts from Grignard reagents ROH + R ′ Mg X R ′ H + Mg(OR) X Stronger acid

Relative acidities

weaker acid

H 2 O 〉 ROH 〉 NH 3 〉 RH

Relative basicities OH - 〉 OR - 〉 NH 2 - 〉 R Rule: one compound is shown to be a stronger acid than another by its ability to displace the second compound from salts. A-H + B- M + B - H + A -M + Alcohols are weaker acids than water as we mentioned above, an alkoxide is not prepared by the reaction of alcohols with sodium hydroxide because of the presence of water in the aqueous solution of sodium hydroxide. + ROH + H O ROH + OH 2

2

Alkoxide are prepared by reaction of alcohols with the active metal itself (Na, K) in the absence of water. 1 ROH + Na RO - Na + + H2 2 Alkoxides are useful reagent, they are powerful bases-stronger than hydroxide.

6.10 0 Preparation of Alcohols: 1-O Oxymercuraation-demeercuration: Alkenes react with w mercurric acetate inn the presennce of waterr to give hydroxymercuurial compo ounds whichh on reduuction yieldd alcohols.

Alkkene

merccuricacetate

Alcohol (Marrkovnikov rule)

2- hydroborati h ion-oxidatiion:

Isobutylene

3- Grignard G synthysis:

4- Hydrolysis H o alkyl hallides: of

utylalcohol (11o) Isobu

Reaaction of A Alcohols:  1-R Reaction of o alcoholss with hyd drogen haalides. Acid d catalysiss Thee hydroxyl group g in a poor leavingg group. Thiis is accomp plished by thhe presencee of acid wh hich protonaate the hydroxyl h grroup and coonvert it to good g leavingg group.

a- The reaaction is cataalyzed by accids. Even thhough the aqqueous hydrrogen halidees are themselves stronng acids, thee presence of o additionaal sulfuricc acid speedds up the forrmation of alkyl a halidess. b Rearranngement of the bt alkyl grroup occurs except, 1o alcohol. a

c- The ordder of reactivvity of alcoohols towardd HX is 3o > 2o > 1o < CH C 3

Sn2 mechanism: m most 1o alcohhols and methaanol

2-Fromation of alkyl sulfonates: Sulffonic acids, ArSO3H, are a related too sulfuric accid and are strong acids. This anioons, the sulffonates, are weaak bases andd hence goood leaving group. g − ArSO H ArSO + H + 3

Strong accid

3

Sulfonatee anion Weak base b

As a result, alkkyl sulphonaates undergoo nucleophiilic substituttion (and elimination) iin much thee same mannner as alkyl a halidess.

Likee alkyl haliddes, alkyl suulfonates arre readily made m from allcohols.

6.15 5 Oxidatio on of Alco ohols: Thee oxidation of o an alcohool involves the t loss of one o or moree hydrogenss (α-hydrogeens) from th he carbon bearring the –OH H group. A prrimary alcoohol containns two α-hyddrogens, andd can eitherr loss one off them to foorm an aldeh hyde,

b of them m to form caarboxylic accid Or both

A seecondary alcohol can loose its only α-hydrogenns to form a ketone.

A teertiary alcohhol containss no α-hydroogens and iss not oxidizzed. (Ann acidic oxiddizing agentt can, howevver, dehydrrate the alco ohol to an allkane and thhen oxidize this).

Of the t many reagents that can be suedd to oxidize alcohols, we w can consider only thhe most com mmon ones, thosse containinng Mn (VII)) or Cr (VI)). Oxiddation of prrimary alcohhols to carbboxylic acidds is usually accomplishhed by use oof potassium m perm manganate.

+1 +7 -8 R RCH 2 OH + K Mn O 4

1o alcohol

purple

ROO − K + + MnnO 2 + KOH H solublle in water H+

RCO OOH A carbooxylic acid innsoluble in watter Oxiddation of alcohols to thhe aldehyde or ketone stage s is usuaally accompplished by thhe use of Cr (VI) as K2Cr C 2O7 or CrO O3.

Prim mary alcohools can be coonverted onnly to aldehyyde by the use u of special reagent ccalled pyridiinium chloorochromatee .

: ::Ethers s:: 6.16 6 Structurre and nomenclature of Ethe ers : Etheers are comppounds withh the generaal formula R-O-R, R Ar-O O-R or Ar-O O-Ar.

If thhe two groupps are identtical, the ethher is said too be symmeetrical (e.g.,, diethyletheer…..), if diifferent, unsyymmetricall (tert-butylm metylether)).

6.17 7 Physical al properties of Ethers :

Etheer have dipoole momentt because thhe C-O-C boond angle iss not 180o. this t weak poolarity don'tt affect to a high exteend the boiliing points of o ethers. Etheers can form m hydrogen bonding wiith water soo they have comparablee solubility in water to alcohols wiith the same moleccular weightt.

6.18 8 Structurre and nomenclature of Ethe ers : A nuumber of syymmetrical ethers are prepared p by the reaction ns of the coorrespondingg alcohols with w sulfuricc acidd. Thiss reaction iss a kind of dehydration d n.

Alcoohols can undergo anotther kind off dehydratioon, involvin ng eliminatioon, to give aalkenes.

Dehhydration to ethers rather than to alkenes in coontrolled by y choice of the t reactionn conditions. For exampple:

Etheers formatioon by dehyddration is ann example of o nucleophiilic substituution with allcohol playiing two rolees: the protonated p alcohol is thhe substratee, and the seecond moleccule of alcoohol in the nnucleophile. The reactioon coulld be either Sn1 or Sn22.

On standing s in contact witth air, most aliphatic etthers are con nverted slow wly into unsstable perox xides:

The presence of peroxides, although in i only low concentratiions are veryy dangerouss, since they y can cause viollent explosioon during distillation d thhat normallyy follow ex xtraction witth ether. d color with the ether thhough shakiing with an The presence of peroxides is indicatedd by formattion of a red m sulfate annd potassium m thiocyanaate; the perooxides oxid dizes ferrouss ion aqueeous solutioon of ferrouus ammonium to feerric ion; whhich react with w thiocyaanate ion to give blood--red color of o the compllex.

Perooxides couldd be removeed by distilllation with concentrateed H2SO4 orr by washingg with soluttion of ferroous ion (which reduuce peroxiddes).

6.19 9 Preperation of Ethers :

Williamson n synthesiis: 1.W In thhe Williamsson synthesis of ethers an alkyl haalide (or sub bstitution alkkyl halide) is allowed to t react withh sodiium alkoxidde.

For example:

Reaaction involvves nucleopphilic substitution of alkkoxide ion for f halide ioon; it is sim milar to the formation f off alcoohols by treaatment of allkyl halidess with aqueoous hydroxide.

2.A Alkoxymerrcuration--demercurration:

6.21 Reaction ns of Ethe ers, Cleavaage by accids

Reaactivity of HX H :

m: Sn1 Mechanism

Sn2 Mechanism m:

HI

> HBr > HCl