Reactions of Alcohols, Phenols, Aldehydes and Ketones

Reactions of Alcohols and Phenols

OBJECTIVES 

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Understand the properties and reactions of alcohol and phenol Enumerate and give the processes involving alcohol and phenol Compare and contrast aldehydes and ketones in terms of their structures and properties

 Alcohols and Phenols 

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Alcohol is is a compound that has a hydroxyl group (-OH) bonded to a saturated, sp3-hybridized carbon atom. It can can be cla lass ssif ifiied as prim primar ary, y, se seco cond ndar ary, y, and tertiary according to the number of alkyl groups directly bonded to the alcohol carbon. Phenol, on the other hand, has a hydroxyl group bonded to an aromatic ring. Its IUP AC name is benzenol.

Solubility in Water COMPOUND Ethyl alcohol n-butyl alcohol sec-butyl alcohol tert-butyl alcohol Benzyl alcohol Glycerol Phenol

SOLUBILITY  SOLUBLE SOLUBLE SOLUBLE SOLUBLE INSOLUBLE SOLUBLE INSOLUBLE

Solubility in Water 

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Increases with of  increasing number hydroxyl groups. Solubility of monohydric alcohols decreases with increasing molecular weight. Branched isomers are more soluble than unbranched isomers.

Solubility in Water Since alcohols can form hydrogen bond with water, low-molecular weight  alcohols are water-soluble. However, as the molecular weight of an alcohol increases, the proportion of it  that is hydrocarbon increases . The alcohol becomes more like an alkane , less like water, and less soluble in water.

Test for Alcohols and Phenols (Acidity and Basicity) Compound

Reaction w/ litmus paper

Ethyl alcohol

Blue to red

Phenol

Blue to red

Reaction w/ pH paper 6 5

Phenols are more acidic than alcohols because phenoxide the anion is resonance-stabilized by the aromatic ring.

Test for Alcohols and Phenols (Acidity and Basicity) Sharing the negative charge over the ring increases the the stab stabililit ity y of the the phen pheno oxide xide an anio ion n and thus increases the tendency of the corresponding phenol to dissociate. The The char charac acte teri rist stic ic prop proper erty ty that that diff differ eren enti tiat ates es phenols from alcohols is acidity. Phenols are weakly acidic and can be neutralized by sodium hydroxide.  Alcohols are less acidic than phenols and are not  neutralized by sodium hydroxide.

Reaction with Sodium n-butyl alcohol

1st 

sec-butyl alcohol Tert-butyl alcohol Phenol

2nd 3rd 4th

 Alcohols react with sodium metal to yield alkoxide salts that are themselves strong bases. 2ROH + 2Na



2RONa + H2o

Reaction with Acetyl Chloride 

Ester formed when acetyl chloride reacts with alcohol and phenol evident  via formation of a top layer, which usually accompanied by the evolution of  heat. ROH + CH3COCl  CH3COOR + HCl  ArOH + CH3COCl  CH3COO Ar + HCl

Lucas Test 

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Lucas reagent  is a mixture of concentrated HCl and anhydrous zinc chloride. Test for relative rates of  alkyl halide formation Test to deduce structure of  alcohols Formation of a chloride from alcohol is indicated by the cloudiness that appears when the separates from the chloride solution.

Lucas Test Reactivity is measured by the time required for the cloudiness to appear Tertiary alcohol  reacts readily at  room temperature

Secondary alcohol  reacts within five minutes Primary alcohol  does not react  appreciably at  room temperature

Lucas Test  Althou though gh the al alccoho ohols ar are e soluble ble in the the Lucas cas reagent, the alkyl halide formed is not.  As tiny droplets of the alkyl halide form, the solution becomes cloudy, thus providing a visual way to the reactions progress. Tert Tertia iary ry al alco coho hols ls re reac actt al almo most st inst instan anta tane neou ousl sly y at  room temperature, and secondary alcohols react in 5 to 15 minutes when heated. Primary alcohols are the slowest, requiring several hours of reaction time, even with heat.

Oxidation One of the most valuable reactions of  alcohols is their oxidation to yield carbonyl compounds by a formal loss of H 2 from the carbon to which the oxygen is attached: CH3CH2-OH  CH3 -HC=O Common oxidizing agents: Chromic acid (H2CrO4) KMnO4 in acid or basic medium K2Cr2O7

Oxidation Primary alcohols yield aldehydes or carboxylic acids

Secondary alcohols yield ketones Tertiary alcohols do not normally react  with oxidizing agents.

Iodoform Test Test for the presence of methyl group attached to a functional carbon. Reagent: NaOH + I2 Structure of alcohol: R



NaOI (sodium (so dium hypo hypoiod iodite ite)

CH3 C

OH

H

Positive result: yellow precipitate of iodoform (CHI3).

Bromine Water Test The bromine in water test is a qualitative test for the presence of  phenol.

FeCl3 

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Test

The positive test result is evident by the formation of blue, green, purple, red-brown, or intense red complexes. This This is an exce excellllen entt test test for for diff diffe ere rent ntia iati tin ng alco al coho holl fro rom m phe heno nols ls.. This is also a test for the presence of an enol group. This test is best carried out in the presence of  pyridine for greater accuracy of result.

FeCl3 3

OH

phenol

+3

Test N

+

FeCl3

Fe(OAr) Fe(OAr)3

colored complex

+3

+

NH

Cl-

Reactions of  Aldehydes and Ketones

 Aldehydes Aldehydes and and Ketones General formula for Aldehydes:

General formula for Ketones:

O R

C

O

H

R

C

R

 Aldehydes Aldehydes and and Ketones 

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Aldehydes and ketones are structurally very similar; both have a carbon-oxygen double bond called a ca carb rbo ony nyll gr grou oup p. They differ in that aldehydes have at  least  one hydrogen atom bonded to the carbonyl group, whereas in ketones the carbonyl is bonded to two carbon atoms.

 Aldehydes Aldehydes and and Ketones 

Characteristic reactions of aldehydes and ketones:    

Addition reactions across the double bond Reduction Condensation Hal alof ofor orm m re reac acti tion on

Oxidation Reaction (Benedicts Test) 

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Preparation: Heat a solution of  17.3 g sodium citrate and 10 g anhydrous sodium carbonate ion 80 mL of distilled water until the salts dissolved. Dissolve 1.73 g of hydrated copper sulfate in 10 mL of water and add this solution to the sol soluti ution of  sodium citrate and sodium carbonate slowly with stirring. Dilute the solution to 100 mL by adding dist di stil ille led d wa wate ter. r.

Oxidation Reaction (Benedicts Test) 

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Benedicts test is one the classical tests employed in the determination of the presence of aldehyde. of aldehyde. Benedict solution can oxidize a variety of  com compoun pounds ds an and d prec precip ipit itat atio ion n of the the cupro uprous us oxide is a pos positi itive ve ind indica icatio tion. n. Aliphatic aldehydes usually produced yellow to orange (yellow, yellowish green, red) precipitate or suspension, sometime greenish in blue solution is observed.

Oxidation Reaction (Benedicts Test) This test is negative for most ketones and aroma ar omati tic c al alde dehyd hydes es.. O

O

+ Cu2O(s) + 2H2O

+2 + 2Cu + 4OH

R

H

aldehyde

R

OH

carboxylic acid

Oxidation Reaction (Chromic Anhydride Test) Preparation  Dissolve 25 g chromic anhydride in 25 mL concentrated su sulf lfur uriic ac aciid.  Pour this suspension slowly with stirring to 75 mL of distilled water.  Cool the solution at  room temperature before using.

Oxidation Reaction (Chromic Anhydride Test) 

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The basis of this test is the reduction of  chr chromiu omium m (IV) IV), which hich is ora rang nge e in color olor,, to chromium (III), (III), which green in color. The positive result for aldehyde is evident by the formation of  an opaque blue green or an intense blue to green suspension at a slower rate. If the unknown is an aliphatic aldehyde, initial cloudiness after 5 seconds is observed and the blue  green suspension forms after 30 seconds.

Oxidation Reaction (Chromic Anhydride Test) Aromatic aldehydes took longer time to react, about 30  90 seconds. Ketones give a negative test and thus, for this reason this is one of the tests used to distinguish aldehydes from ketones.

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O

O 3 R

+ 2 CrO3 + 3H2SO4

C

y

ora g - r

+ 3 H2O + Cr2(SO4)3

C R

H al

3

OH

arboxyli a i

opaqu blu - gr or i t r s blu to gr

Oxidation Reaction (To ollens llens (Silver Mirror) Test) Preparation  Clean a container with 10% NaOH and place 2 mL of AgNO3 and add a drop of  10% NaOH.  Add 2% ammonia, drop by drop, with constant  shaking, until the precipitate of  silver oxide  just dissolves. sens nsiitive reagent, it is  In order to obtain a se necessary to avoid excessive addition of  ammonia.

Oxidation Reaction (To ollens llens (Silver Mirror) Test) 

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This test is used to distinguish aldehydes from ketones ketones.. Also known as silver mirror test because of its positive test result is evident by the formation formation of metallic metallic silver on the surface surface of the test tube. In some cases, formation of granula larr gray or black precipitate is observed as a positive test result.

Oxidation Reaction (To ollens llens (Silver Mirror) Test) 

Ketones result.

usually

give

negative H +

H N

H

O

O

+

C

R

test 

2Ag( 2Ag Ag( Ag(NH3)2OH

2Ag(s 2Ag(s))

H

aldeh de

silver nitrate in ammoni m h droxide ide

silver mirror

+

R

C

H

O ammoni m s alt of c ar ox lic acid

+

H2 O

+

NH3

Oxidation Reaction (Fehlings Test for Aldehydes Aldehydes) Preparation  Mix 2.5 mL of each of the following solutions: 

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Fehlings A. Diss sso olve 17.32 7.32g g of  hydrated copp copper er su sulf lfat ate e crys crysta tals ls in 200m 200mL L of wate waterr and dilute to 250mL. Fehlings B. Dissolve 86.5g of  sodium potassium tartrate (Roc (Roche helllle ess sa salt lt)) an and d 35g of sodium hydroxide in 100 mL of water and dilute to 250mL.

Oxidation Reaction (Fehlings Test for Aldehydes Aldehydes) 

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The basis of this test is the reduction of copper (II) to copper (I). For aliphatic aldehydes, formation of brick red, yellow or yellowish green color of precipitate of  Cu2O indicates the positive test results. In the the cour coursse of the the re reac acti tion on,, the the Feh ehlling ngs reagent reduced the deep blue copper (II) ion complex to brick red copper (I) oxide.

Oxidation Reaction (Fehlings Test for Aldehydes Aldehydes) 

Aromatic aldehydes give a negative test  result. For ketones, this test give a negative test result. O

O R

+

C

2 Cu+2

y

+

C

Cu2O(s)

OH

H al

R

blu

arb arboxyli a i

bri k r , y llow or y llowi owis gr

Reaction of the Carbonyl Group (Condensation Reaction) 2,4  Dinitrophenylhydrazine 

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The positive test results for most  aldehydes and ketones are evident by the formation of insoluble solid of   dinitrophenylhydrazones. The color of the precipitate formed may also indicate the structure of the aldehydes and ketones.

Reaction of the Carbonyl Group (Condensation Reaction)  Aromatic aldehydes and ketones (highly con jugated) tend to form orange to red prec precip ipit itat ate e while hile al alip ipha hati ticc an and d unco uncon n jugated aldehydes and ketones tend to form yellow precipitate. The precipitate formed is sometimes oily which become crystalline after some time. Formation of oily precipitate of   dini dinitr trop ophe heny nylh lhyd ydrraz azo one ness is char chara acter cteris isti ticc of  numerous ketones.

Reaction of the Carbonyl Group (Condensation Reaction) This maybe use to differentiate aldehydes from ketones. In some cases, mild heating is required in order for the reaction to be visible, but avoid over overhe heat atiing be beca caus use e it may caus cause e oxid oxidat atiion and in effect give false positive test result. R' O R

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N

NH2

+

C R'

ldehyde or ketone

O

-

+

+

N

N

O

O

-

2,4-dinitrophenylhydr zine  

C

NH

NH

O

O

+

O

+

N

N

O

O

-

2,4-dinitrophenylhydr zone  

R

Reaction of the Carbonyl Group (Addition Reaction) Sodium bisulfite addition 

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Bis isul ulfa fate te ions ons re reac actt with with al ald deh ehyd yde e and ketone to form crystalline addition products. Formation of solid product is an evidence of positive test result.

Reaction of the Carbonyl Group (Addition Reaction) Compounds having active carbonyl carbons do give also a positive test result.  Among those compounds are methyl ketones, and low molecular weight cyclic ketones of up to eight eight-- membered cyclic ketone. O R

+

C R

aldehyde or ketone

H

O Na

S

OH

R

OH +

O

SO3 Na

sodium bisulfite addition complex

Reaction of the Carbonyl Group (Ha aloform loform Rxn / / Iodoform Test) 

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This is to test the presence of the meth me thyl yl gr grou oup. p. Compounds containing the carbonyl group will react with halogen in the presence of a base by a substitution type of reaction.

Reaction of the Carbonyl Group (Ha aloform lofform Rxn / lo / Iodoform Test) 

An alpha halo ketone is formed and when NaOH and iodine are used, a yellow precipitate of iodoform is produced. Positive test result is evident by the formation of yellow precipitate of  iodoform with a medicinal odor or foulsmell.

Reaction of the Carbonyl Group (Ha aloform lofform Rxn / lo / Iodoform Test) The following give a positive test result:  Acetaldehyde Conjugated aldehydes such as acrolein and and furf furfur ural al Dihyd Di hydoxy oxy com compou pounds nds Met ethy hyll ke keto tone ne Secondary alcohol with methyl group adjacent to the carbon bearing the hydr hydrox oxyl yl grou group p

Reaction of the Carbonyl Group (Ha aloform loform Rxn / / Iodoform Test) CH3

OH

+

CH H3C

CH3

a tone

4I2

+

+

6NaOH

+

Na O O

CHI3

+

Iodofor Iodofor (yellow solid)

5NaI

+

5H2O