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Lecture (8) Hydrocarbons derivatives: The vast majority of organic molecules contain elements in addition to carbon and hydrogen. However, most of these substances can be viewed as hydrocarbons derivatives; e.g.: alcohols and phenols, aldehydes and ketons and carboxylic acids. V-Alcohols 1-Introduction: Alcohols are compounds of the general formula R-OH, where R is any Alkyl group; the group may be primary, secondary or tertiary; it may be open chain or cyclic, it may contain a double bond, or an aromatic ring. 2-Classification of alcohols: Alcohols are classified as primary (1°), secondary (2°) or tertiary (3°); depending on the number of carbon substituents bonded to the hydroxylbearing carbon: H R R R C OH H Primary alcohol (1°) H C OH R R secondary alcohol (2°) C OH R tertiary alcohol (3°) 3-Nomenclature of alcohols: Alcohols are named by many system using common names, carbinol system or IUPAC system (the name of hydrocarbon from which they are derived, followed by the suffix "-OL". IUPAC system: 1- Select the longest carbon chain containing the hydroxyl group, and replace the –e ending of the corresponding alkane with "OL" 2-Number the carbon; begin at the end nearer to the hydroxyl group. 3-Number all substituents according to their position; and write in alphabetical order. 45 Examples: OH (1)-CH3 C CH2CH2CH3 1 2 3 4 2-methyl-2-pentanol (IUPAC) 5 CH3 CH3 (2)-CH3 C OH 2-methyl-2-propanol (IUPAC) Tert-butyl alcohol (common) Tri methyl carbinol (carbinol) CH3 Alcohols containing two hydroxyl groups are called { Dihydric alcohols};[GLYCOLS]: Examples: CH2 ─ CH2 1,2-Ethandiol (IUPAC) OH Ethylene glycol (common) OH CH2 CHCH3 1,2-Propandiol(IUPAC) OH OH Propylene glycol (common) Alcohols containing three hydroxyl groups are called {Trihydric alcohols}: OH OH OH CH2- CH- CH2 Glycerine (1,2,3-trihydroxyPropane) 46 4-Prepatration of Alcohols: A-Reduction of carbonyl compounds (ketones or aldehydes) O H2/Pt (1)-CH3 CH2 C ─H CH3CH2CH2─OH Propanal High pressure n-propyl alcohol (1°) CH3 CH3 H2/Pt (2)-CH3─C=O Acetone CH3─CH─OH Isopropyl alcohol (2°) B-Hydration of Alkenes: H H CH3CH=CH2 cold H2SO4 CH3─ C─CH3 H2O CH3─C─CH3+H2SO4 ∆ OSO3H OH Iso propyl alcohol C-By Grignard reactions with aldehydes or ketones: RMgX/H2O (1)-HCHO RCH2OH + MgXOH Formaldehyde (1°) RMgX/H2O (2)-R'CHO R'RCHOH + MgXOH Aldehyde (2°) RMgX/H2O (3)-R'R"CO R'R"RCOH + MgXOH Ketone (3°) 47 5-Physical Properties of Alcohols: Alcohols, like water, are both weakly basic and weakly acidic. Alcohols contain the very polar –OH group. This group contains hydrogen attached to the very electronegative element OXYGEN, and therefore permits hydrogen bonding R─O…..H-O-R H The boiling point of alcohols is higher than hydrocarbons of the same molecular weight. Lower alcohols are miscible with water. 6-Chemical properties of alcohols: The chemical properties of an alcohol are determined by its functional group, the hydroxyl group reactions of an alcohol can involve the breaking of either of two bonds, C─OH bond, with removal of the (OH) group or the (O-H) bond with removal 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. Difference in the structure of the group cause differences in reactivity and some of the more important reactions of alcohols are listed below: A-Reactions based on the substitution of the hydroxyl group (R─OH bond cleavage): Reaction with hydrogen halides: ROH + HX R-X + H2O e.g.: HCL/ZnCL2 CH3CH2CH2CH2CH2OH CH3CH2CH2CH2CH2CL +H2O n-Pentylalcohol Heat n-Pentylchloride Reactivity towards ROH (HX) HI>HBr>HCL Reactivity towards HX (ROH) 3°>2°>1° 48 Examples: 25° (1)- (CH3)3COH + HCL Tertiary Alcohol (3°) (CH3)3 CCL + H2O ZnCL2 (2)- (CH3)2CHOH + HCL Secondary alcohol (2°) (CH3)2CHCL + H2O ZnCL2/heat (3)- (CH3)CH2OH + HCL Primary alcohol (1°) CH3CH2CL + H2O B-Reactions based on the substitution of the hydrogen of the hydroxyl group (RO-H bond cleavage): 1-Reaction with active metals (as acid) RO-H + M RO-M+ + 1/2 H2 (M=Na,K,Mg,AL) Reactivity of ROH: The rate of reaction is higher with primary and decreases with secondary then to tertiary alcohol. Na CH3CH2OH CH3CH2O-Na+ + 1/2 H2 Ethyl alcohol sodium ethoxide 2-Ester formation CH3CH2O-H + CH3COOH Ethyl alcohol acetic acid : H + H3C-COOCH2CH3 + H2O ethylacetate C-Dehydration: Elimination of water to yield alkenes: Conc.H2SO4(-H2O) ─C ─C─ ─C = C─ (Reactivity of ROH 3°>2°>1°) 49 D-Oxidation of alcohols :( Differentiation between alcohols) The oxidation of an alcohol involves the loss of one or more hydrogen (αhydrogens) from the carbon bearing the –OH group. The kind of product that is formed depends upon how many of this α-hydrogen the alcohol contains, that is, upon whether the alcohol is primary, secondary or tertiary. (1)-Primary alcohol contains two α-hydrogens and can lose one of them to form an aldehyde. KMnO4 RCH2OH RCHO RCOOH Alcohol (1°) aldehyde carboxylic acids (2)-Secondary alcohol can lose its only α-hydrogen to form a ketone K2Cr2O7 R2CHOH R2CO Alcohol (2°) (3)-Tertiary alcohol contains no α-hydrogen and is not oxidized. KMnO4 R3COH no oxidation Oxidation of primary alcohols to carboxylic acids is usually by potassium permanganate. Oxidation of secondary alcohols to ketone by potassium dichromate. 7-Relative acids: The alkyl group makes an alcohol less acidic than water, the bigger the alkyl group, the less acidic the alcohol. 50