1 Chapter 4 Alcohols and Alkyl Halides 4 1 Functional Groups o Major Classes of Organic Compounds Class Alcohol Alkyl halide Amine Epoxide Generalized Abbreviation OH X halogen NH2 Ether Nitrile Nitroalkane Sulfide Thiol Aldehyde Ketone Carboxylic Acid Acyl halide Acid anhydride Ester Amide O C N NO2 S SH O ll CH O ll C O ll CX O ll CO O ll CN o Alcohols resemble water in their acidity o Carboxylic acid is a weak acid but stronger acid than alcohol Carbonyl containing C O compounds rank among the most abundant and biologically significant classes of naturally occurring substances 4 2 IUPAC Nomenclature of Alkyl Halides 2 o Functional class nomenclature the alkyl group and the halide are designated as seperate words o The alkyl group is named bases of its longest continuous chain beginning at the carbon to which the halogen is attached CH3F methyl fluoride CH3CH2CH2CH2CH2Cl pentyl chloride cyclohexyl iodide o Substitutive nomenclature treats the halogen as a halo substituent on an alkane chain The carbon chain is numbered in the direction that gives the substituted carbon the lower number Is listed alphabetically CH3CH2CH2CH2CH2F 1 fluoropentane 4 3 IUPAC Nomenclature of Alcohols o The alkyl group that bears the hydroxyl substituent OH and then adding alcohol as a separate word o Substitutive names of alcohols are developed by identifying the longest continuous chain that bears the hydroxyl group and replacing the e ending of the corresponding alkane by the suffix ol CH3CH2OH ethyl alcohol or ethanol trans 2 methylcyclopentanol 4 4 Classes of Alcohols and Alkyl Halides o Alcohols and alkyl halides are classified as primary secondary and tertiary according to the degree of substitution of the carbon that bears the functional group o Where these functional groups are placed determine what the properties of the alcohols are o Halogens and oxygen are more electronegative than carbon and the carbon halogen bond in alkyl halides and the carbon oxygen bond in alcohols are polar 4 5 Bonding in Alcohols and Alkyl Halides o The carbon that bears the functional group is sp3 hybridization on in alcohols and alkyl halides The halogen connected to sp3 hybridization carbon by a bond C F C Cl C Br C I Carbon oxygen and carbon halogen bonds are polar covalent bonds and carbon bears a partial positive change in alcohols and in alkyl halides Polar molecules 3 4 6 Physical Properties of Alcohols and Alkyl Halides Intermolecular Forces o Boiling Point Dipole dipole attractive force Two molecules of a polar substance experience a mutual attractive between the positively polarized region of one molecules and the negatively polarized region of the other Permanent dipoles have stronger intermolecular forces than alkanes Intermolecular forces importance can be seen by shape and size The attractive forces in ethanol are usually strong Hydrogen bonding 15 20 times weaker than covalent bonds organic compounds that involve hydrogen bonding are oxygen and nitrogen The hydrogen must be bonded to a strongly electronegative element in order for the bond to be polar enough to support hydrogen bonding Among alkyl halides the boiling point increases wit increasing size of the halogen In OH oxygen is the hydrogen bond acceptor and the hydrogen is the hydrogen bond donor Polarizablility Induced dipole induced dipole attractions are strongest in alkyl iodides weakest in alkyl fluorides Chloromethane 24 C Dichloromethane 40 C Trichloromethane 61 C Tetrachloromethane 77 C Increasing the number of halogens also increases the induced dipole induced dipole attractive forces and therefore the boiling point o Solubility in Water All alkyl halides are insoluble in water Low molecular weight alcohols are soluble in water o Density Alkyl fluorides and chlorides are less dense and alkyl bromides and iodides more dense than water When the alkyl halide is a fluoride or chloride it is the upper Alkyl halide is a bromide or an iodide In these cases the alkyl layer and water is the lower halide is the lower layer Liquid alcohols have densities of approximately 0 8 g mL and are therefore less dense than water Increasing halogenation increases density 4 7 Preparation of Alkyl Halides from Alcohol and Hydrogen Halides 4 o The reaction of alcohols with hydrogen halides yields alkyl halides o R OH H X R X H OH o The order of reactivity of the hydrogen halides parallel their acidity o Tertiary alcohols are observed to be the most reactive and primary HI HBr HCl HF alcohols the least reactive temperature Tertiary needs low temperature and primary need a high Tertiary alcohols are converted to alkyl chlorides CH3 3COH HCl CH3 3CCl H2O Secondary and primary alcohols do not react with HCl at rates fast enough to make the preparation of the corresponding alkyl chlorides a method of practical value HBr 4 8 Mechanism of the Reaction of Alcohols with Hydrogen Halides H2O Hammond s Postulate the SN1 mechanism o The reaction of an alcohol with a hydrogen halide is a substitution o The mechanism is the step by step pathway of bond cleavage and bond formation that leads from reactants to products o Hammond s postulate if two states are similar in energy they are similar in structure o A halogen replaces a hydroxyl group as a substitution on the carbon o An elementary step is one that involves only one transition state o Step 1 Protonation exothermic Proton transfer The alcohol act as a Br nsted base Proton transfer to an alcohol gives an alkylozonium ion ROH2 In all reactions the first step gives us a alkylozonium ion 5 Potential Energy Diagram The transition state is not a stable structure and the bonds are partially formed and broken o Step 2 dissociation Carbocation Formation high energy sp2 The alkyloconium ion dissociates to a molecule of water and carbocation an ion that contains a positively charge carbon Because this step is endothermic the products of it are placed higher in energy than the reactants The transition state is closer in energy to the carbocation Potential Energy Diagram 6 o Step 3 Lewis acid Lewis base reaction chloride attaches to the carbocation The Lewis bases that react with electrophiles are called nucleophiles Potential Energy Diagram The step is exothermic it leads from the carbocation intermediate to the stable isolated products of the reaction The activation energy for this step is small the bond formation between a positive ion and a negative ion occurs rapidly The transition state for this for this step involves