Aspects 6/4/2016Dr Seemal Jelani Chem-2401

Structure of Alcohols The hydroxyl groups of alcohols are good hydrogen bonding donors and acceptors 6/4/2016Dr Seemal Jelani Chem-2402

The functional group of an alcohol is an OH (hydroxyl) group bonded to an sp3- hybridized carbon 6/4/2016Dr Seemal Jelani Chem-2403

The oxygen atom of an alcohol is also sp3 hybridized Two sp3 hybrid orbitals of oxygen form sigma bonds to atoms of carbon and hydrogen, and the remaining two sp3 hybrid orbitals each contain an unshared pair of electrons 6/4/2016Dr Seemal Jelani Chem-2404

The measured C-O-H bond angle in methanol is 108.9°, very close to the perfectly tetrahedral angle of 109.5°. 6/4/2016Dr Seemal Jelani Chem-2405

6/4/2016Dr Seemal Jelani Chem-2406 Physical properties

Detail Physical properties Dipole-dipole interaction The attraction between the positive end of one dipole and the negative end of another Hydrogen bonding The attractive interaction between a hydrogen atom bonded to an atom of high electronegativity (most O or N) 6/4/2016Dr Seemal Jelani Chem-2407

6/4/2016Dr Seemal Jelani Chem-2408 and a lone pair of electrons on another atom of high electronegativity (again, most commonly O or N). δ– H δ+ O δ+ C H H H

6/4/2016Dr Seemal Jelani Chem-2409 Why alcohols are considered as Polar compounds Due of the presence of the polar-OH group, alcohols are polar compounds, with partial positive charges on carbon and hydrogen and a partial negative charge on oxygen

6/4/2016Dr Seemal Jelani Chem The attraction between the positive end of one dipole and the negative end of another is called dipole-dipole interaction.

6/4/2016Dr Seemal Jelani Chem When the positive end of one of the dipoles is a hydrogen atom bonded to O or N (atoms of high electronegativity) and the negative end of the other dipole is an O or N atom, the attractive interaction between dipoles is particularly strong and is given the special name of hydrogen bonding.

Polarity of the C-O-H bond in alcohol 6/4/2016Dr Seemal Jelani Chem-24012

6/4/2016Dr Seemal Jelani Chem δ+ δ− δ+ δ−

6/4/2016Dr Seemal Jelani Chem There is extensive hydrogen bonding between alcohol molecules in the pure liquid It shows the association of ethanol molecules by hydrogen bonding between the partially negative oxygen atom of one ethanol molecule and the partially positive hydrogen atom of another ethanol molecule.

6/4/2016Dr Seemal Jelani Chem-24015

Effect of Hydrogen bonding The presence of additional hydroxyl groups in a molecule further increases the extent of hydrogen bonding, as can be seen by comparing the boiling points of hexane (bp 69°C), 1-pentanol (bp 138°C), and 1,4-butanediol (bp 230°C) 6/4/2016Dr Seemal Jelani Chem-24016

Boiling Points and Solubilities in Water of Five Groups of Alcohols and Hydrocarbons of Similar Molecular Weight Mol wt BP S/water CH 3 OH Methanol Infinite CH 3 CH 3 Ethane Insoluble CH3CH2OH Ethanol Infinite CH3CH2CH3 Propane Insoluble CH3CH2CH2OH Infinite 1-Propanol CH3CH2CH2CH Insoluble Butane 6/4/2016Dr Seemal Jelani Chem-24017

CH3CH2CH2CH2OH g/100 g 1-Butanol CH3CH2CH2CH2CH3 Pentane Insoluble HOCH2CH2CH2CH2OH Infinite 1,4-Butanediol CH3CH2CH2CH2CH2OH g/100 g 1-Pentanol CH3CH2CH2CH2CH2CH Insoluble Hexane 6/4/2016Dr Seemal Jelani Chem-24018

Acidity and Basicity of Alcohols Alcohols can function as both weak acids (proton donors) and weak bases (proton acceptors) In dilute aqueous solution, only methanol (pKa 15.5) is more acidic than water. 6/4/2016Dr Seemal Jelani Chem-24019

CH3-O-H + H-O-H CH3-O : + H-O-H H 6/4/2016Dr Seemal Jelani Chem-24020

Acidity is associated not only with the tendency of compound to yield hydrogen in H 2 O but also to accept an electron pair to form a covalent bond. 6/4/2016Dr Seemal Jelani Chem-24022

The strengths of weak acids are measured on the pKa scale. The smaller the number on this scale, the stronger the acid is.

The less the charge is delocalized, the less stable the ion, and the weaker the acid.

General Rule 1. Acidity increases as the electronegativity increases Example: H – CH 3 < H – NH 2 < H – OH < H – F < H – SH < H – Cl 2. Within the Family, acidity increases as the size increases Example: H – F < H – Cl < H – Br < H – I

Ethanol has about the same acidity as water Higher-molecular-weight, water soluble alcohols are slightly weaker acids than water Thus, although alcohols have some acidity, they are not strong enough acids to react with weak bases such as sodium bicarbonate or sodium carbonate 6/4/2016Dr Seemal Jelani Chem-24026

In a polar solution, acidity of alcohols is lower, than acidity of water due to the less efficient solvation of the alkoxides (steric reason).

For simple alcohols, such as methanol and ethanol, acidity depends primarily on the degree of solvation and stabilization of the alkoxide ion by water molecules 6/4/2016Dr Seemal Jelani Chem-24028

The negatively charged oxygen atoms of the methoxide and ethoxide ions are almost as accessible for solvation as the hydroxide ion is; therefore, these alcohols are about as acidic as water. 6/4/2016Dr Seemal Jelani Chem-24029

pKa Values for Selected Alcohols in Dilute Aqueous Solution* Stronger Acid Compound Structural Formula pKa Hydrogen chloride HCl 27 Acetic acid CH3COOH 4.8 Methanol CH3OH 15.5 Water H2O /4/2016Dr Seemal Jelani Chem-24030

Ethanol CH3CH2OH Propanol (CH3)2CHOH 17 2-Methyl-2- (CH3)3COH 18 propanol Weaker acid 6/4/2016Dr Seemal Jelani Chem-24031

Conclusion To summarize, when trying to predict the mechanisms of reactions involving an hydroxy group, you need to keep in mind that it is both a weak acid and a weak base, so consider adding a proton or taking a proton away in the initial steps of mechanisms when there is a strong acid or base present, respectively. 6/4/2016Dr Seemal Jelani Chem-24032

In addition, an important mechanistic theme in many of the reactions of alcohols is that the -OH group, a poor leaving group, reacts with protons or a variety of strong electrophiles to create –OH 2 + or analogous group, a much better leaving group, enabling subsequent substitution or elimination reactions to take place. 6/4/2016Dr Seemal Jelani Chem-24033

6/4/2016Dr Seemal Jelani Chem Reactions of Alcohol

35 Oxidation and Reduction Ways of recognizing oxidation/reduction reactions: Oxidation and reductions always occur togetherOxidationReduction An atom loses electrons An atom gains electrons An atom gains a bond to oxygen An atom loses a bond to oxygen An atom loses a bond to hydrogen An atom gains a bond to hydrogen

36 Reactions of Alcohols This same definition can also be applied to the oxidation of alcohols by potassium dichromate (K 2 Cr 2 O 7 ). The oxidation requires that there are hydrogens to be removed on the carbon to which the hydroxyl is bound

Reaction of Alcohols with Active Metals Alcohols react with Li, Na, K, and other active metals to liberate hydrogen and form metal alkoxides In this oxidation/reduction reaction, Na is oxidized to Na + and H + is reduced to H 2 2 CH 3 OH + 2 Na 2 CH 3 O _ Na + + H 2 sodium methoxide 6/4/2016Dr Seemal Jelani Chem-24037

The sulfuric acid removes the alcohol from the air into a liquid solution. The alcohol reacts with potassium dichromate to produce: * chromium sulfate * potassium sulfate * acetic acid * water Silver nitrate is used as a catalyst 6/4/2016Dr Seemal Jelani Chem-24038

6/4/2016Dr Seemal Jelani Chem-24039

Application: Breathalyzer 6/4/2016Dr Seemal Jelani Chem-24040

41 Reactions of Alcohols The oxidation of primary (1°) alcohols is a way for preparing aldehydes and carboxylic acids. The oxidation of secondary (2°) alcohols is a way for preparing ketones. The oxidation of tertiary (3°) alcohols does not occur because there are not hydrogens attached to the carbon to which the hydroxyl is attached

42 Reactions of Alcohols and Thiols

43 Reactions of Alcohols In biological reactions the coenzyme NAD + is often used as the oxidizing agent. The NAD + takes the electrons away from alcohols to produce aldehydes, carboxylic acids and ketones.

44 Reactions of Alcohols Example The oxidation of malate to oxaloacetate that occurs in the citric acid cycle:

45 Another reaction the dehydration of alcohols to produce alkenes

46 Preparations of Alcohols, Like the complement hydration reaction, dehydration can also produce multiple products.

47 Preparations of Alcohols Multiple products occur whenever there are a different number of hydrogen atoms attached to the two carbons that are on either side of the carbon to which the hydroxyl is attached. There is a rule that can be used to predict which of the two products is predicted to be the major product.

In a dehydration of an alcohol, the hydrogen will be removed from the neighboring carbon atom that carries the fewest hydrogen atoms. 6/4/2016Dr Seemal Jelani Chem-24048

49 Examples of dehydration of alcohols

50 Reactions of Alcohols with Aldehydes and Ketones Aldehydes and ketones can react with alcohols to form hemiacetals, hemiketals, acetals and ketals. This is because carbohydrates are rich in aldehydes, ketones and alcohols