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L 10 alcohols-structure_nomenclature_classification_etc_pch217_2013_2014


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L 10 alcohols-structure_nomenclature_classification_etc_pch217_2013_2014

  1. 1. 1 Organic Chemistry Course Number: PCH 1120-217 Lecture # 10 Sunday September 29, 2013 Alcohols: Structure, Nomenclacture, Classification, Phys ical Properties, Basicity & Acidity and Preparation of Alcohols Prof. Oludotun Phillips Room # 2-81, 2nd Floor Pharmacy Building Email:
  2. 2. 2 Alcohols: Learning Objectives At the end of the class students should be able to:  discuss the structures and properties of alcohols.  classification of alcohols.  discuss the nomenclature (common and IUPAC names) of alcohols.  describe the physical properties of alcohols, basicity and acidity of alcohols.  discuss the different methods for the preparation of alcohols.
  3. 3. 3 • The functional group of an alcohol is an -OH group bonded to an sp3 hybridized carbon – bond angles about the hydroxyl oxygen atom are approximately 109.5° • Oxygen is also sp3 hybridized – two sp3 hybrid orbitals form sigma bonds to carbon and hydrogen – the remaining two sp3 hybrid orbitals each contain an unshared pair of electrons Alcohols - Structure
  4. 4. 4 • IUPAC names – the parent chain is the longest chain that contains the - OH group – number the parent chain in the direction that gives the - OH group the lower number – change the suffix -e to -ol • Common names – name the alkyl group bonded to oxygen followed by the word alcohol Alcohols - Nomenclature
  5. 5. 5 • Examples: Eth an o l (E th y l alc oh ol) 1-P ro p an ol (Pro p y l alco h o l) 2-Prop an o l (Is op ro p y l alco h o l) 1-B u tan o l (B u ty l alco h o l) OH OH OH OH 2-B u tan ol (s ec-B u tyl alco h o l) 2-M e th y l-1-p ro p an o l (Is ob u ty l alc oh ol) 2-M e th y l-2-p rop an o l (t ert-B u ty l alco h o l) OH Cy clo h e xan o l (Cy clo h e xy l alco h o l) OH OH OH Nomenclature of Alcohols
  6. 6. 6 • Problem: Write the IUPAC name of each alcohol O H CH3 ( CH 2 ) 6 CH2 O H O H (a) (b ) (c) Nomenclature of Alcohols
  7. 7. 7  Polyhydric alcohols: compounds containing…. – two -OH groups are named as diols, – three -OH groups are named as triols, etc. CH3 CH CH2 HO OH CH2 CH 2 OHOH CH2 CH CH 2 OHHOH O 1,2-Eth an e d io l (Eth yle n e gly co l) 1,2-P ro p an e d io l (P ro p y le n e g lyc ol) 1,2,3-Pro p an e trio l (G ly ce rol, G ly ce rin ) Nomenclature of Alcohols
  8. 8. 8  Unsaturated alcohols – the double and triple bond are shown by the infix -en- and -yn- – the hydroxyl group is shown by the suffix -ol – number the chain to give OH the lower number 5 2 OH13 46 tran s- 3-h e xe n e -1-ol (E)-3-h e xe n e -1-ol IUPAC Nomenclature of Alcohols H OH 5-Hexny-1-ol
  9. 9. 9 Alcohols are further classified as: 1. Methyl alcohol (CH3OH) 2. primary (1o)alcohol 2. secondary (2o)alcohol 3. tertiary (3o)alcohol Classification of Alcohols H OH R H R' OH R H R' OH R R" (1o) (2o) (3o) primary secondary tertiary
  10. 10. 10 Alcohols are further classified as: 5. allylic alcohol 6. benzylic alcohol Classification of Alcohols CH3 CH CHCH2 O H H CH3 H OH CH2 O H C-CH3 OH H C-CH3 OH CH3 Benzylic alcohols (1o) (2o) (3o)allylic alcohol (1o primary)
  11. 11. 11  Alcohols are polar compounds  both the C-O and O-H bonds are polar covalent - + + O H H H C H Alcohols – Physical Properties
  12. 12. 12  Alcohols associate in the liquid state by hydrogen bonding.  Hydrogen bonding: the attractive force between a partial positive charge on hydrogen and a partial negative charge on a nearby oxygen, nitrogen, or fluorine atom. – the strength of hydrogen bonding in alcohols is approximately 2 to 5 kcal/mol. – hydrogen bonds are considerably weaker than covalent bonds (for example, 110 kcal/mol for an O-H bond). – hydrogen bonding can have a significant effect on physical properties. Alcohols – Physical Properties
  13. 13. 13 – association of ethanol molecules in the liquid state, with two of the three possible hydrogen bonds to the upper oxygen are shown as follows:. Alcohols – Physical Properties
  14. 14. 14 – alcohols have higher boiling points and are more soluble in water than hydrocarbons CH3 CH2 CH 2 OH CH3 CH2 CH 2 CH3 CH3 OH CH3 CH3 CH3 CH2 OH CH3 CH2 CH 3 CH3 CH2 CH 2 CH2 CH 2 OH HOCH2 CH 2 CH2 CH2 OH CH3 CH2 CH 2 CH2 CH 2 CH3 S tru c tu ral Fo rm u la N am e M o le cu lar W e ig h t (g/m o l) Boilin g P oin t (°C ) S o lu b ility in W ate r m e th an o l 32 65 in f in ite e th an e 30 -89 in s olu b le e th an ol 46 78 in f in ite p ro p an e 44 -42 in s olu b le 1-p ro p an o l 60 97 in f in ite b u tan e 58 0 in s olu b le 1-p e n tan o l 88 138 2.3 g /100 g 1,4-b u tan e d io l 90 230 in f in ite h e xan e 86 69 in s olu b le Alcohols – Physical Properties
  15. 15. 15 Acidity Constants:  According to Bronsted-Lowry definitions:  an acid is a hydrogen ion donor.  a base is a hydrogen ion acceptor. The strength of the acid is usually compared measuring their degree of ionization in water. Relative Acidity of Alcohols
  16. 16. 16 Most alcohols are about the same or slightly weaker acids than water – pKa = -logKa – aqueous solutions of alcohols have the same pH as that of pure water –  Alcohols are neutral compounds! CH 3 O H O H H [ CH3 O - ] [ H3 O + ] [ CH3 O H] CH3 O H O H H + K a = + + = 3.2 x 10 - 16 p K a = 15.5 Acidity of Alcohols
  17. 17. 17 In the presence of strong acids, the oxygen atom of an alcohol can behave as a weak base – proton transfer from the strong acid forms an oxonium ion – thus, alcohols can function as both very weak acids and weak bases – Alcohols can lose a proton to a very strong base, e.g. NaOH + ROH  RO- Na+ + H2 CH3 CH2 - O-H H O H H O H H H2 SO4 CH 3 CH 2 -O H H CH 3 CH 2 -O H H HH O H Eth y lo xon iu m ion (p K a -2.4) • • H y d ron iu m ion (p K a -1.7) Eth an o l + + + ++ • • + + + Basicity of Alcohols
  18. 18. 18 – pKa values for several low-molecular-weight alcohols ( CH3 ) 3 CO H ( CH3 ) 2 CHO H CH3 CH 2 O H H 2 O CH3 OH CH3 CO O H H Cl C om p o u n d p K a -7 15.5 15.7 15.9 17 18 4.8 h y d ro ge n ch l o ri d e ace ti c aci d m e th an o l w ate r e th an ol 2-p ro p an o l 2-m e th y l -2-p rop an o l S tru c tu ral Form u l a S tro n g e r aci d W e ak e r aci d *A l s o gi v e n f o r c om p ari s o n are p K a v al u e s f o r w ate r, ace ti c aci d , an d h y d ro ge n ch l o ri d e . Acidity of Alcohols
  19. 19. 19  Alcohols can be prepared by the following methods: i. Fermentation of carbohydrates ii. Hydration of alkenes iii. Nucleophilic substitution reaction iv. Reduction of aldehydes and ketones Preparation of Alcohols
  20. 20. 20 Alcohols can be prepared by: i. Fermentation of carbohydrates:  Fermentation is the decomposition of complex organic compounds into simpler compounds by the action of biological catalysts known as enzymes.  Fermentation of glucose give ethanol:… Preparation of Alcohols C6H12O6 CH3CH2OH enzymes Glucose ethanol (a sugar) (an alcohol)
  21. 21. 21 Alcohols can be prepared by: ii. Hydration of alkenes:  Treatment of alkenes with water in the presence of a strong acid as a catalytic results in the addition of water to the double bond (hydration reaction):…. + H2 OCH2 CH2 CH3 CH2 OH Preparation of Alcohols Cyclohexane cyclohexanol H+ Strong acid (conc. H2SO4) + O HH2 O H+ Strong acid (conc. H2SO4)Ethylene Ethanol b. a.
  22. 22. 22 Alcohols can be prepared by: iii. Nucleophilic Substitution Reactions:  Reaction of an alkyl halide in hydroxide ion is a nucleophilic substitution reaction.  Primary alcohols are prepared by heating primary alkyl halides in aqueous sodium hydroxide (NaOH), this reacion proceed by SN2 mechanism:…  Since 2o and 3o alkyl halides are likely to give elimination reaction products, this method is not very useful for preparing alcohols. Preparation of Alcohols CH3 CH2 CH2 Br CH3 CH2 CH2 O H + NaBr+ NaO Haq heat 1-Bromopropane 1-Propanol (a 1o alkyl halide) (a 1o alcohol)
  23. 23. 23 Alcohols can be prepared by: iv. Reduction of Aldehydes and Ketones: a. Alcohols are prepared from carbonyl compounds by reduction reactions in which hydrogen atoms are added to the carbonyl group by NaBH4 or LiAlH4 followed by H2O or dilute acid hydrolysis:….. Preparation of Alcohols R C H2 C HO R C H2 C H2 O H NaBH4 Or LiAlH4An Aldehyde A 1o alcohol R C OC H3 R C HC H3 OH A Ketone A 2o alcohol NaBH4 Or LiAlH4 Reagents: Sodium borohydride: (NaBH4) Lithium aluminum hydride (LiAlH4)
  24. 24. 24 Alcohols can be prepared by: iv. Reduction of Aldehydes and Ketones: b. By Catalytic Hydrogenation involves the addition of Hydrogen to aldehydes or ketones under pressure to produce 1o and 2o Alcohols respectively:….. Preparation of Alcohols R CH2 CHO R CH2 CH2 OH+ H2 Ni Pressure An Aldehyde hydrogen A 1o alcohol R CO CH3 R CHCH3 O H + H2 A Ketone hydrogen A 2o alcohol Reagents: Nickle (Ni); Hydrogen gas (H2) Ni Pressure
  25. 25. 25 Alcohols can be prepared by: iv. Reduction of Aldehydes and Ketones: c. By Grignard addition to an Aldehyde or Ketone followed by H2O or dilute acid hydrolysis to produce 1o, 2o and 3o Alcohols respectively.  Grignard Reagent is prepared by reacting Magnesium metal with an Alkyl halide in diethyl ether as solvent. Preparation of Alcohols R X R Mg X+ Mg diethyl ether (as solvent) Alkyl halide Magnesium Grignard reagent R = CH3; CH3CH2; or Ph; X = Cl; Br; I In this reaction, the “R” of Grignard reagent acts as Nucleophile
  26. 26. 26 Alcohols can be prepared by:  Formaldehyde is reduced to 1o alcohol:..  Aldehydes are reduced to 2o alcohols:…  Ketones are reduced to 3o alcohols:… Preparation of Alcohols H C H O H C-H R" O- + M gX + R "M gX H C-H R" O H Formaldehyde Grignard reagent Magnesium salt of alcohol 1o alcohol R C O C H3 R C-C H3 R " O - + M g X + R "M g X R C-C H3 R " O H R' CH O R' C-H R" O- +MgX + R"M gX R' C-H R" OH H2O H+ Aldehyde Grignard reagent Magnesium salt of alcohol 2o alcohol H2O H+ H2O H+ Ketone Grignard reagent Magnesium salt of alcohol 3o alcohol