General Chemistry II
General Chemistry II CHM 204
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Date Created: 11/01/15
Chapter 13 Alcohols Phenols Ethers and Thioethers Earlier you saw that an alcohol is an organic molecule containing an 0H group bound to a saturated carbon atom Phenols are benzene rings that have attached 0H groups Ethers are organic molecules with an oxygen bound to two organic groups The pre x thio means sulfur containing and usually means an oxygen atom has been replaced by a sulfur atom Thus thioethers are ethers where sulfur has substituted for the oxygen The term thioethers is uncommon however and the term sulfides is more commonly used 131 Occurrence Types and Names of Alcohols The term alcohol has an interesting origin Like many words in science beginning with al this term has an Arabic origin Al means the Thus alchemy is the chemistry The last two syllables come from the word for an early eye shadow made of antimony powder The word for the process by which this powder was made is very similar to the word for the powder itself The process is essentially distillation Medieval Europeans learned of distillation through the operation of making liquor Of course the active ingredient of liquors is ethanol but at the time the name of the process by which ethanol was concentrated was given to the chemical itself Hence the original name of ethanol was alcohol Molecules containing one or more 70H groups are common in nature Their use is so widespread that many have common names For example OH OH HO OH HoOH CH3OH CH3CH20H wood alcohol grain alcohol rubbing alcohol glycerin methyl alcohol ethyl alcohol isopropyl alcohol ethylene glycol glycerol methanol ethanol 2propanol l2ethanediol 123propanetriol Since the vast majority of alcohols have their 70H groups attached to alkanes their names are most easily generated by using those alkanes as their base 1 Find the longest carbon chain which includes the carbon to which the 0H groups is attached 2 Name the molecule as if it were an alkane Start counting from the end closest to the 70H group L V Change the na e to ol and put the 70H location prior to that chain name using numbers if required to remove ambiguity The examples just given also include their systematic names When more than one 70H functionality is present use the endings diol triol etc Common names for the alcohols shown on p l are also provided you The common name for alcohols is generated simply by taking the name of the alkane chain see p 11 of Chapter 11 notes and adding alcohol to the end The isopropyl alcohol molecule shown on p 1 provides an example This is how common names are usually generated For example CH3CHClCH3 replace the OH in isopropyl alcohol with Cl is isopropyl chloride Alcohols fall into 3 major structural category types These categories are important because the types of reaction an alcohol may undergo depends on what else is attached to its carbon If the carbon to which the 70H group binds also binds to 1 carbon atom the alcohol is designated primary l Ifonly two carbons are attached to that carbon the alcohol is secondary 2 If three carbons are attached the alcohol is tertiary 3 The R groups below represent any fragment attaching through a carbon atom including aromatic units 1 IF RCHZOH RCOH RCOH H Ru primary l secondary 2 tertiary 3 Please note that this classi cation scheme does not affect the naming of an alcohol all it does is allow you decided quickly what kinds of reactions the alcohol in question may participate in 132 Physical Properties of Alcohols Alkanes interact with one another exclusively through London Forces p 161 of textbook The addition of an 70H group makes the molecule polar allowing for dipoledipole interactions and more importantly provides sites for hydrogen bonding pp 165169 of textbook Recall that hydrogen bonding is an unusually strong dipoledipole interaction This means that alcohols tend to stick together much more strongly than alkanes and this affects their physical properties Before going further please remember that the effects of the 70H group decrease as the alkane chain becomes longer Thus any hydrogen bonding effects in methanol are much larger than those in lpentanol This was illustrated in the table of alcohol solubilities in water on p 7 of the Chapter 11 notes A major effect that hydrogen bonding causes is a significant increase in boiling point On that same page of the notes is a table ofthe boiling points of3 different species As you can see adding 0H groups causes substantial increases in boiling point Your book does a similar table 132 p 408 where it keeps the molecular weights roughly the same but the major point in either case is that hydrogen bonding causes large increases in boiling points Stronger intermolecular forces make it more difficult to separate the molecules Again this effect drops off as the molecules get bigger Other properties not of biochemical interest are also affected For example the Viscosity of small alcohols is much higher than the Viscosity of alkanes Viscosity is the resistance to ow by a liquid In other words at the same temperature alcohols are thicker than alkanes It is dif cult but possible to see this comparing ethanol and pentane the lightest alkane that is a liquid at room temperature You can see this by simply swirling a ask containing each If you have ever seen glycerin though it is as thick as warm maple syrup and it is only a little heavier than pentane 133 Chemical Properties of Alcohols Two types of reactions alcohols are prone to engage in are discussed in this chapter One is essentially the reverse of addition of water to an alkene This is called dehydration the loss of water from an alcohol The other is called oxidation This reaction results in loss of hydrogen and the conversion of the C0 single bond to a double bond Dehydration Your book gives you as an example the dehydration of ethanol p 410 CH3CH20H H2CCH2 H20 At this point you may well remember that the book used the same reaction in reverse as an illustration of addition reactions p 376 textbook p 7 Chapter 12 notes H2CCH2 H20 CH3CH20H The same reaction shown in both directions On the surface this is a contradiction Fortunately a chemical principle from CHM 203 steps in and explains this apparent contradiction Can you guess it In Chapter 6 p 16365 you learned about equilibria This is an extremely important topic that we will revisit several more times this semester and you need to review it if you have forgotten it Le Chatelier s principle reminds us that when a system at equilibrium is disturbed it will adjust so as restore the equilibrium Although the temperatures at which these reactions are conducted are different this is not the principal cause of the direction of reaction For a hydrolysis reaction alcohol formation there is a very large excess of water present to ensure that all of the alkene is converted to alcohol In the case of dehydration either water is removed as the reaction progresses or the equilibrium already so favors alkene production that the small amount of water produced doesn t really affect the position of the equilibrium very much This reaction proceeds by a path essentially the reverse of the water addition path presented in the previous chapter Given that it is probably not surprising that for interior 0H groups the final location of the CC bond depends on the branching of the alkane For example TH L Hc al St M major product minor product In general the double bond will locate on the carbons with the most branching although frequently there will be some of the other product present As your book notes when this reaction occurs biochemically only one product forms which one depends on the enzyme This is because enzymes have stringent spatial requirements that allow the reactant alkene to bind in only one way before the reaction begins This ensures only one product comes out and that product may not be the one preferred by standard organic methods Oxidation In an oxidation one of two things will happen Either an external reagent will remove two hydrogen atoms resulting in the conversion of a CO bond to a CO bond or the reagent will transfer an oxygen atom to the molecule added as a 0 Sometimes both reactions occur although in a stepwise fashion The reverse of these processes is called a reduction Oxidations that occur via the removal of 2 hydrogen atoms are frequently called dehydrations although the end product is still an oxidation product It is important to remember that an organic oxidation is formally the same as an inorganic oxidation We ll see how they are the same in the next few paragraphs For reasons that will soon be obvious only 10 and 2 alcohols can be oxidized to other organic compounds Although if you provide harsh enough conditions they can be oxidized all the way to C02 and H20 When the oxidant 0 reacts with an alcohol it effectively pulls the hydrogen atom bound to the oxygen off It also takes a hydrogen bound to the carbon bound to the 70H off as well The net effect is that it pulls two electrons out of the bonding system Since adding electrons negative charges is a reduction the reverse process removing electrons must be an oxidation This can be see pictorially Q CQ H20 U Now if the reactant is a primary alcohol we should wind up with an aldehyde As it turns out aldehydes are usually quite unstable to further oxidation What this means is that the aldehyde reacts more rapidly with oxidant than does the parent 10 alcohol Thus if one mixes one equivalent of primary alcohol with one equivalent of oxidant instead of getting one equivalent of aldehyde one obtains a mixture containing a substantial amount of the corresponding carboxylic acid In other words this is a poor way to make aldehydes but a good one to make carboxylic acids Secondary alcohols react to form ketones Examples of each type of reaction include H O T SCI 0 c amp C H H H H H QH H methanol formaldehyde formic id OH l CH ampgt C H3C I 3 H3C CH3 H 2propanone 2propanol acetone Two common oxidants are permanganate ion MnO439 and dichromate ion Cr207239 As you learned in Chapter 4 p 85 whenever an oxidation occurs a reduction must also proceed Does it here Let s look at the balanced chemical equation for the oxidation of lpropanol to propanoic acid on p 413 of your book Can you assign oxidation numbers to each element below 3 CH3CH2CH20H 2 Cr207239 16 Ht gt 3 CH3CH2C02H 4 Cr3 11 H20 In this reaction a Cr in Cr207239 is reduced to Cr3 when the lpropanol is oxidized to propanoic acid The oxidation number of the carbon bound to oxygen goes from 1 in lpropanol to 3 in propanoic acid This is the oxidation As you ve probably guessed strong oxidants like MnO439 can t exist in your body We have to use enzymes to accomplish the same types of reactions in vivo As mentioned earlier because enzymes are very speci c they can oxidize an alcohol to an aldehyde with no carboxylic acid byproduct This is a reaction manifested in human bodies across college campuses on a nightly basis Since most of you are not yet 21 years old you will of course not know of this from personal experience but you may have heard of the hangover from
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