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ALCOHOLS

ALCOHOLS. Aspects. Structure of Alcohols. The hydroxyl groups of alcohols are good hydrogen bonding donors and acceptors. 180.9 0 109.3 0 The functional group of an alcohol is an OH (hydroxyl) group bonded to an sp3-hybridized carbon. The oxygen atom of an alcohol is also sp3 hybridized

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ALCOHOLS

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  1. ALCOHOLS Aspects Dr Seemal Jelani Chem-240

  2. Structure of Alcohols The hydroxyl groups of alcohols are good hydrogen bonding donors and acceptors Dr Seemal Jelani Chem-240

  3. 180.9 0109.3 0 • The functional group of an alcohol is an OH (hydroxyl) group bonded to an sp3-hybridized carbon Dr Seemal Jelani Chem-240

  4. 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 Dr Seemal Jelani Chem-240

  5. The measured C-O-H bond angle in methanol is 108.9°, very close to the perfectly tetrahedral angle of 109.5°. Dr Seemal Jelani Chem-240

  6. Physical properties Dr Seemal Jelani Chem-240

  7. 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) Dr Seemal Jelani Chem-240

  8. 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 Dr Seemal Jelani Chem-240

  9. 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 Dr Seemal Jelani Chem-240

  10. The attraction between the positive end of one dipole and the negative end of another is called dipole-dipole interaction. Dr Seemal Jelani Chem-240

  11. 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. Dr Seemal Jelani Chem-240

  12. Polarity of the C-O-H bond in alcohol Dr Seemal Jelani Chem-240

  13. δ− δ+ δ+ δ− Dr Seemal Jelani Chem-240

  14. 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. Dr Seemal Jelani Chem-240

  15. Dr Seemal Jelani Chem-240

  16. 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) Dr Seemal Jelani Chem-240

  17. Boiling Points and Solubilities in Water of Five Groups of Alcohols and Hydrocarbons of Similar Molecular Weight Mol wt BP S/water • CH3OH Methanol 32 65 Infinite • CH3CH3Ethane 30 289 Insoluble • CH3CH2OH Ethanol 46 78 Infinite • CH3CH2CH3 Propane 44 242 Insoluble • CH3CH2CH2OH60 97 Infinite 1-Propanol • CH3CH2CH2CH3 58 0 Insoluble Butane Dr Seemal Jelani Chem-240

  18. CH3CH2CH2CH2OH 74 117 8 g/100 g 1-Butanol • CH3CH2CH2CH2CH3 Pentane 72 36 Insoluble • HOCH2CH2CH2CH2OH 90 230 Infinite 1,4-Butanediol • CH3CH2CH2CH2CH2OH 88 138 2.3 g/100 g 1-Pentanol • CH3CH2CH2CH2CH2CH386 69 Insoluble • Hexane Dr Seemal Jelani Chem-240

  19. 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. Dr Seemal Jelani Chem-240

  20. .. .. .. - .. + • CH3-O-H + H-O-H CH3-O : + H-O-H .. .. .. H Dr Seemal Jelani Chem-240

  21. Structural Effectson Acidity

  22. Acidity is associated not only with the tendency of compound to yield hydrogen in H2O but also to accept an electron pair to form a covalent bond. Dr Seemal Jelani Chem-240

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

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

  25. General Rule 1. Acidity increases as the electronegativity increases Example: H – CH3 < H – NH2 < 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

  26. 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 Dr Seemal Jelani Chem-240

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

  28. 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 Dr Seemal Jelani Chem-240

  29. 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. Dr Seemal Jelani Chem-240

  30. 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 15.7 Dr Seemal Jelani Chem-240

  31. Ethanol CH3CH2OH 15.9 • 2-Propanol (CH3)2CHOH 17 • 2-Methyl-2- (CH3)3COH 18 propanol Weaker acid Dr Seemal Jelani Chem-240

  32. 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. Dr Seemal Jelani Chem-240

  33. 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 –OH2+or analogous group, a much better leaving group, enabling subsequent substitution or elimination reactions to take place. Dr Seemal Jelani Chem-240

  34. Reactions of Alcohol Dr Seemal Jelani Chem-240

  35. Oxidation Reduction 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 Oxidation and Reduction • Ways of recognizing oxidation/reduction reactions: • Oxidation and reductions always occur together

  36. Reactions of Alcohols • This same definition can also be applied to the oxidation of alcohols by potassium dichromate (K2Cr2O7). The oxidation requires that there are hydrogens to be removed on the carbon to which the hydroxyl is bound

  37. 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 H2 2CH3 OH + 2 Na 2CH3 O_Na + + H2 sodium methoxide Dr Seemal Jelani Chem-240

  38. 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 Dr Seemal Jelani Chem-240

  39. Dr Seemal Jelani Chem-240

  40. Application: Breathalyzer Dr Seemal Jelani Chem-240

  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.

  48. In a dehydration of an alcohol, the hydrogen will be removed from the neighboring carbon atom that carries the fewest hydrogen atoms. Dr Seemal Jelani Chem-240

  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

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