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3.9 Reactions of alcohols 1.Combustion 2. Dehydration 3. Formation of ethers 4.Oxidation 1 The same as hydrocarbons undergo combustion in the presence of oxygen, alcohols burn with oxygen too. Combustion CH3OH + 2O2 CO2 + 2H2O + Heat 2 Dehydration of Alcohols to form Alkenes Alkenes can add water to yield alcohols. In a reverse reaction, alcohols can lose water molecule when they are heated with an acid catalyst such as H2SO4. H OH H+, heat H-C-C-H H H alcohol (ethanol) H-C=C-H + H2O H H alkene (ethene) 3 Dehydration of Alcohols to form Alkenes During the dehydration of an alcohol, a H– and – OH are removed from adjacent carbon atoms of the same alcohol to produce a water molecule. A double bond forms between the same two carbon atoms to produce an alkene. H OH H+, heat H-C-C-H H H alcohol (ethanol) H-C=C-H + H2O H H alkene (ethene) 4 Saytzeff’s Rule The dehydration of a secondary alcohol can result in the formation of either of two products. Saytzeff’s rule state that the major product is the one that results when the hydrogen is removed from the carbon atom with the smallest number of hydrogen atoms. “THE POOR GET POORER” rule 5 Saytzeff’s Rule The major product is the one that results when the hydrogen is removed from the carbon atom with the smallest number of hydrogen atoms. “The poor get poorer” 2H 3H H +H2O H+ Heat 2-Butene (major product: 90%) +H2O OH 2-Butanol 1-Butene (minor product: 10%) 6 THE MOST SUBSTITUTED C=C IS THE MAJOR PRODUCT Formation of Ethers ANOTHER KIND OF DEHYDRATION Ethers form when the dehydration of alcohols occurs at lower temperatures in the presence of an acid catalyst. R—OH + HO—R’ H+ Low Heat R—O—R’ + H2O The components of water are removed from two molecules: an H– from one alcohol and the –OH from another. 7 Formation of Ethers ANOTHER KIND OF DEHYDRATION CH3—OH + HO—CH3 H+ Low Heat CH3—O—CH3 + H2O 8 Oxidation of Alcohols The reactions of alcohols have a central role in organic chemistry because alcohols can be converted to many of the other functional groups. Reduction is • a gain of electrons • a less positive oxidation number • a gain of hydrogen atoms • the loss of oxygen atoms • the loss of bonds to oxygen. Oxidation is • a loss of electrons • a more positive oxidation number • a loss of hydrogen atoms • the addition of oxygen atoms. • more bonds to oxygen 2 Bonds to O 1 Bond to O Alkane Alcohol (1°) OH [O] CH3 CH3 CH3 CH2 3 Bonds to O Aldehyde [O] O CH3 C Carboxylic acid O [O] H CH3 C OH [O] CO2 + H2O 9 Oxidation numbers • Each C-H is a -1 • Each C-O is a +1 • Sum total C-H and C-O to determine Oxidation Number 10 Oxidation numbers • Each C-H is a -1 • Each C-O is a +1 What are the oxidation numbers for CH3OH CH2O HCO2H Hint: Write the expanded formula to determine the number of bonds to C 11 OXIDATION REACTIONS •Primay •Seconday •Tertiary ALCOHOLS 12 OXIDATION REACTIONS Oxidation of Primary Alcohol Primary alcohol [O] aldehyde [O] carboxylic acid Example: oxidation of methanol CH3OH CH2O HCO2H 13 Oxidation of Secondary Alcohol Secondary alcohol [O] Ketone Double bonds forms Two hydrogen atoms removed OH R1 CH R2 [O] O R1 C R2 2° Alcohol H2O Ketone R1, R2: alkyl groups 14 Metabolic example C H 3 O H O C H C O H C H 3 O O C C O H Lactic acid dehyrogenase Lactic acid Pyruvic acid 15 OH CH3 O [O] CH2 CH3 C H OH CH3 CH2 ethanol + 6+ Cr orange [O] [O] Oxidation of Alcohol in the body CO2 + H2O O CH3 CO H Acetic acid + 3+ Cr green Breathalyzer test for suspected drunk drivers 16 Oxidation of Tertiary Alcohol Tertiary alcohol [O] No reaction Only one hydrogen atom, not the two required to be removed OH R1-C-R2 [O] NR R3 3° Alcohol R1, R2, R3: alkyl groups 17 Question What is the dehydration product when 3-methyl-2-butanol reacts with acid in the presence of heat? a. trans-2-butene b. 2-methyl-2-butene c. trans-2-methyl-2-butene d. 3-methyl-1-butene Answer: “b”. The alkene forms so that the carboncarbon double bond is more substituted. This is Saytzeff's rule. 18 Question What is the product of this reaction? a. b. c. d. a carboxylic acid a ketone an aldehyde no product forms Answer: “d”. Tertiary alcohols cannot oxidize. 19 Question The hydration of 1-methylcyclopentene gives a. b. c. d. 1-methylcyclopentanol. 2-methylcyclopentanol. 3-methylcyclopentanol. None of these form. Answer”a”. This reaction is the addition of water to a double bond. Do you remember Markovnikov's rule? The hydrogen from water adds to the end of the double bond with the most hydrogens. 20 Question What product forms and what type of reaction is this? a. The reaction is an oxidation and the product is b. The reaction forms ether and the product is c. The reaction is an oxidation and the product is d. The reaction forms ether and the product is Answer: “b”. 21