Alcohols Classification

Transcription

1 Alcohols Alcohols are compounds of the general formula ROH, where R is any alkyl or substituted alkyl group. The group may be primary, secondary, or tertiary; it may be open-chain or cyclic; it may contain a double bond, a halogen atom,or an aromatic ring. For example: Note:- 1- All alcohols- contain the hydroxyl (- OH) group, which, as the functional group, 2- Variations in structure of the R group may affect the rate at which the alcohol undergoes certain reactions, and even, in a few cases, may affect the kind of reaction. 3-Compounds in which the hydroxyl group is attached directly to an aromatic ring are not alcohols; they are phenols, Classification We classify a carbon atom as primary, secondary, or tertiary according to the number of other carbon atoms attached to it An alcohol is classified according to the kind of carbon that bears the OH group: 1

2 Certain substituents may affect reactivity in such a way as to make an alcohol of one class resemble the members of a different class; benzyl alcohol, for example, though formally a primary alcohol, often acts like atertiary alcohol. We shall find that these variations, too, are consistent with the structures involved. Nomenclature Alcohols arc named by three different systems. For the simpler alcohols the common names,, are most often used. These consist simply of the name of the alkyl group followed by the wordalcohol. For example: It is sometimes convenient to name alcohols by the carbinol system. According to this system, alcohols are considered to be derived from methyl alcohol, CH3OH, by the replacement of one or more hydrogen atoms by other groups. We simply name the groups attached to the carbon bearing the OH and then add the suffix -carbinol to include the C OH portion: Finally, there is the most versatile system, the IUPAC. The rules are: (1) Select as the parent structure the longest continuous carbon chain that contains the OH group \ then consider the compound to have been derived from this structure by replacement of hydrogen by various groups. The parent structure is known as ethanol, propanol, butanol, etc., depending upon the number of carbon atoms; each name is derived by replacing the terminal -e of the corresponding alkane name by -ol. 2

3 (2) Indicate by a number the position of the OH group in the parent chain, generally using the lowest possible number for this purpose. (3) Indicate by numbers the positions of other groups attached to the parent chain. Alcohols containing two hydroxyl groups are called glycols. They have both common names and IUPAC names. Physical properties 1-This compounds are pola OH group,this group contains hydrogen attached to the very electronegative element, oxygen, and therefore permits hydrogen bonding 2-the relatively low melting points 3- alcohols is that they boil so high much higher than hydrocarbons of the same molecular weight, and higher,even,than many other compounds of considerable polarity increase in boiling point with increasing carbon number,decrease in boiling point with branching 4- The solubility behavior of alcohols also reflects their ability to form hydrogen bonds. In sharp contrast to hydrocarbons, the lower alcohols are miscible with water. 3

4 5- alcohol molecules are held together by the same sort of intermodular forces as water molecules, there can be mixing of the two kinds of molecules:the energy required to break a hydrogen bond between two water molecules or two alcohol molecules is provided by formation of a hydrogen bond between a water molecule and an alcohol molecule. 6- the first three primary alcohols are miscible with water; n-butyl alcohol is soluble to the extent and the higher alcohols still less. Industrial source (a) Hydration of alkenes (b) Fermentation of carbohydrates Fermentation of sugars by yeast, the oldest synthetic chemical process, is still of enormous importance for the preparation of ethyl alcohol and certain other alcohols In industry ethyl alcohol is widely used as a solvent for lacquers, varnishes,perfumes, and flavorings; as a medium for chemical reactions.in addition, it is an important raw material for synthesis Ethyl alcohol is the alcohol of "alcoholic" beverages. For this purpose it is prepared by fermentation of sugar from a truly amazing variety of vegetable sources. Preparation of alcohols 1. Oxymercuration-demercuration addition of water to the double bond,with orientation-markovnikov 4

5 2. Hydroboration-oxidation. With the reagent diborane,(bh 3 ) 2,alkenes undergo hydroboration to yield alkylboranes, R 3 B, which on oxidation give alcohols.the hydroboration-oxtdation process gives products to anti-markovnikov addition of water to the carbon-carbon double bond 5

6 3. Grignard synthesis 6

7 4. Hydrolysis of alkyl halides 5. Aldol condensation Catalytic hydrogenation of,j8-unsaturated aldehydes and ketones yields saturated alcohols, addition of hydrogen occurring both at carbon-carbon and at carbon-oxygen double bonds. It is for the purpose of ultimately preparing saturated alcohols that the aldol condensation is often carried out. For example, /i-butyl alcohol and 2-ethyl-l-hexanol are both prepared on an industrial scale in this way: 7

8 Unsaturated alcohols can be prepared if a reagent is selected that reduces only the carbonyl group and leaves the carbon-carbon double bond untouched; one such reagent is sodium borohydride, NaBH4. 6-Reduction of carbonyl compounds. Aldehydes can be reduced tojjrimary alcohols, and ketones to secondary alcohols, either by catalytic hydrogenation or by use of chemical reducing agents 'ike lithium aluminum hydride, LiAlH4. 7-Reduction of acids and esters Conversion of alcohols into acids is important because, in general,alcohols are more available than acids.this is not always true,however;long straight-chain acids from fats are more available than are the corresponding alcohols, and here the reverse process becomes important: reduction of acids to alcohols. Lithium aluminum hydride, LiAlH 4, is one of the few reagents that can reduce an acid to an alcohol; the initial product is an alkoxide from which the alcohol is liberated by hydrolysis 8

9 8- Hydroxyiation of alkenes 9-Grignard synthesis of alcohols The Grignard reagent, we recall, has the formula RMgX. the Grignard reagent is its reaction with aldehydes and ketones to yield alcohols. The class of alcohol that is obtained from a Grignard synthesis depends upon the type of carbonyl compound used: formaldehyde, HCHO, yields primary alcohols; other aldehydes, RCHO, yield secondary alcohols; and ketones, R2CO, yield tertiary alcohols 9

10 For example A related synthesis utilizes ethylene oxide to make primary alcoholscontaining two more carbons than the Grignard reagent the organic group becomes attached to carbon and magnesium to oxygen, this time with the breaking of a carbon-oxygen a bond in the highly strained three-membered ring 10

11 Reactions of Alcohols Reactions of an alcohol can involve the breaking of either of two bonds: the C-OH bond,with removal of the OH group;orthe O-H bond, withremoval of H. either kind of reaction can involve substitution, in which a group replaces the OH or -H, or elimination, in which a double bond is formed. 1. Reaction with hydrogen halides. Alcohols react readily with hydrogen halides to yield alkyl halides and water. Note1 The reaction is catalyzed by acids. Even -though the aqueous hydrogen Note2 halides are themselves strong acids, the presence of additional sulfuric acid speeds up the formation of halides. Note3 Rearrangement of the alkyl group occurs, except with most primary alcohols. 11

12 Note4 order of reactivity of alcohols toward HX is allyl, benzyl > 3 > 2 >1 < CH3. Q/write the mechanism of reaction between alcohols and hydrogen halides? alcohol accepts (step 1) the hydrogen ion to form the protonated alcohol, which dissociates(step 2) into water and a carbonium ion; the carbonium ion then combines(step3)with a halide ion (not necessarily the one from step 1) to form the alkyl halide note Primary alcohols do not undergo rearrangement simply because they do not react by this mechanism. Instead, they react by the alternative SN2 mechanism : 2. Reaction with phosphorus trihalides. 12

13 3. Dehydration. Mechanism dehydration involves (1) formation of the protonated alcohol, ROH 2 + (2) its slow dissociation into a carbonium ion (3) fast expulsion of a hydrogen ion from the carbonium ion to form an alkene. Reactivitythe rate of elimjnation depends greatly upon the rate of formation of the carbonium ion,which in turn depends upon its stability.the stability of a carbonium ion, on the basis of inductive effects and resonance,3 > 2 > 1 Orientation the hydrogen ion takes place in such a way as to favor the formation of the more stable alkene.the relative stability of an alkene on the basis of the number of alkyl 13

14 groups attached to the doubly-bonded carbons, and on the basis of conjugation with a benzene ring or with another carbon-carbon double bond. Rearrangement carbonium ion can rearrange,and that this rearrangement seems to occur whenever a1,2-shift of hydrogen or alkyl group can form a more stable carbonium ion 4. Reaction as acids: reaction with active metals. 5. Ester formation 14

15 6. Oxidation tertiary alcohols are not oxidized at all under alkaline conditions. mechanism oxidation reaction Oxidation of secondary alcohols by Cr(VI) is believed to involve (1) formation of a chromate ester, which (2) loses a proton and an HCrO 3 - i n to form the ketone. It is possible that the proton is lost to an oxygen of the ester group in a cyclic mechanism (2a). 15

16 7-Formation of alkyl sulfonates Alcohols react with sulfonyl chlorides to form esters, alkyl sulfonates note Sulfonyl chlorides (the acid chlorides of sulfonic acids)are prepared by the action of phosphorus pentachloride or thionyl chloride on sulfonic acids or their salts : Syntheses using alcohols The alcohols that we have learned to make can be converted into other kinds of compounds having the same carbon skeleton; from complicated alcohols we can make complicated aldehydes, ketones, acids, halides, alkenes, alkynes, alkanes, etc. 16

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18 Alcohols as acids alcohol, acting as a base, can accept a hydrogen ion to form the protonated alcohol ROH 2+ and reactions in which an alcohol,acting as an acid,loses a hydrogen ion to form the alkoxide ion, RO ". the polarity of theo-h bond should facilitate the separation of the relatively positive hydrogen as the ion;viewed differently, electronegative oxygen should readily accommodate the negative charge of the electrons left behind. The acidity of alcohols is shown by their reaction with active metals to formmhydrogen gas, and by their ability to displace the weakly acidic hydrocarbons from their salts With the possible exception of methanol, they are weaker acids than water, but stronger acids than acetylene or ammonia: We may expand our series of acidities and basicities, then, to the following: 18