Chapter 04 Alcohols and Alkyl Halides part 01

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1 hapter 04 Alcohols and Alkyl alides part 01 EM 341: Spring 2012 Prof. Greg ook

2 Functional Groups A functional group is a structural feature in a molecule that has characteristic reactivity. A functional group has similar behavior in every molecule that contains it. arbon-arbon Multiple Bonds Alkenes - double bonds Alkynes - triple bonds Arenes - aromatic rings 2 2 arbon-other atom Single Bonds Alcohols Ethers allides Amines Thiols Sulfides O O 3 Br N 2 S S 3 arbon-oxygen Double Bonds (arbonyls) Aldehyde Ketone Acid Ester Amide Acid alide O O O O O O O O 3 N 2 2

3 Functional Groups A functional group is a structural feature in a molecule that has characteristic reactivity. A functional group has similar behavior in every molecule that contains it. arbon-arbon R Multiple O Bonds R X Alkenes - double bonds Alkynes - triple bonds Arenes - aromatic rings 2 2 alcohols arbon-other atom Single Bonds alkyl halides Alcohols Ethers allides Amines Thiols Sulfides O O 3 Br N 2 S S 3 arbon-oxygen Double Bonds (arbonyls) Aldehyde O Ketone O Acid O Ester O Amide O Acid alide O O O 3 N 2 2

4 Reactions of Functional Groups R O + X R X + O R X + O R O + X R + X X R X + X 3

5 General Reaction asses Addition Reactions - two or more molecules combine to form a new product Elimination Reactions - a molecule breaks apart into two or more molecules Substitution Reactions - part of a molecule is substituted for another group Rearrangement Reactions - A molecule does not gain or lose atoms, but reorganizes its bonds or groups. Also called isomerization. 4

6 General Reaction asses Addition Reactions A + B + 2 cat. Elimination Reactions A + Substitution Reactions B O A B + A + B 3 + O 3 O + cat. + O Rearrangement Reactions A B O heat O 5

7 Alkyl alides Organic halogen compounds are found in many places from common solvents for chemistry and industry, to refrigerants (largely banned for their ozone destruction properties) and natural products common solvents F 2 2 Fs (chlorofluorocarbons) N N natural products (epibatidine) 6

8 Alkyl alide Nomenclature Alkyl halides are often named using common naming (functional class) considering the alkyl group as a substituent on the halide. 3 Br methyl bromide cyclopentyl iodide I 1-methylpropyl chloride 7

9 Alkyl alide Nomenclature IUPA naming is best using the carbon chain as the parent and the halogen as a halo- substituent I 3 Br bromomethane iodocyclopentane 2-chlorobutane Br Br 4-bromo-2-methylhexane 2-bromo-4-methylhexane 8

10 Alcohols Alcohols are often named using common naming (functional class) considering the alkyl group as a substituent on the O functional group. 3 O methyl alcohol O cyclopentyl alcohol O isopropyl alcohol 9

11 Alcohol Nomenclature IUPA naming is best using the carbon chain as the parent and replacing the -ane ending with -anol note: alcohols outrank alkyl substituents 3 methanol O O cyclopentanol O O O 2-propanol 5-methyl-3-hexanol 4-methyl-2-hexanol 10

12 asses of Alkyl alides and Alcohols Alcohols and alkyl halides are classified according to the degree of alkyl substituion on the carbon the functional group is attached O 1-propanol a primary alcohol 2-chloro-2-methylbutane a tertiary halide (chloride) O trans-3-methylcyclopentanol a secondary alcohol 11

13 Properties of Alkyl alides Reactivity is dominated by the polarity of the -X bond µ = 1.9 D diopole moment (Debyes) bond length (Å) bond strength (kcal/mol) fluoromethane 3 -F chloromethane bromomethane 3 -Br iodomethane 3 -I

14 Properties of Alcohol Acohols are polar but have other reactive features R O µ = 1.7 D weakly basic weakly acidic 13

15 Boiling Points of alides and Alcohol 14

16 Intermolecular Forces Induced dipole/induced dipole (London forces, van der Waals attraction) Dipole/induced dipole forces Diplole/Dipole forces 15

17 Intermolecular Forces O R µ = 1.9 D bp -24 µ = 1.7 D bp 65 16

18 ydrogen Bonding At about 5 kcal/mol, -bonds are weaker than covalent bonds but stronger than many other intermolecular forces δ R O δ + R O δ δ + δ + δ O R hydrogen bond hydrogen bond 17

19 Polarizability Polarizability - the ability to distort (polarize) electrons in a bond chloromethane bp -24 F dichloromethane bp 40 F F trichloromethane bp 61 F F F tetrachloromethane bp 77 F F F F F F fluroethane bp -32 1,1-difluoroethane bp -25 1,1,1-trifluoroethane bp -47 hexafluoroethane bp

20 Alcohol Solubility Alcohols are similar to water but the solubility decreases as the hydrocarbon chain increases Alcohol arbons Solubility in Water (g/100 g 2 O) 3 O 1 miscible 3 2 O O O O O miscible miscible O O O 10 insoluble 19

21 Alkyl alide Solubility and Density Alkyl halides are hydrophobic and insoluble in water. Fluorides and chlorides are less dense than water, bromides and iodides more dense But, if it has more than one chlorine, they are more dense less dense more dense more dense more dense 20

22 Preparation of Alkyl alides from alcohols by a substitution reaction R O + X R X + O Alkyl Alcohol ydrogen alide Alkyl alide Water F < < Br < I O < < R reactivity R O R O R < < methyl primary secondary tertiary R R O reactivity 21

23 Preparation of Alkyl alides O Br 120 Br O Br O Br Br Br 25 22

24 Preparation of Alkyl alides an use a combination of halide salt and other Brønsted acids O NaBr 2 SO 4 heat Br 23

25 Preparation of Alkyl alides an use a combination of halide salt and other Brønsted acids NaBr 2 SO 4 O heat ow does the reaction work? Br 23

26 Preparation of Alkyl alides an use a combination of halide salt and other Brønsted acids O NaBr 2 SO 4 heat ow does the reaction work? A reaction mechanism is a detailed step by step description of a chemical transformation showing Br all elementary steps and intermediates 23

27 Thermodynamics The change in energy of a reaction from one side to the other ΔG = -RT ln K eq ΔG = Δ - TΔS Entropy Amount of freedom (or disorder). Greater entropy means more freedom, less entropy means more restricted Gibb's Free Energy total change in energy in the system from reactants to products Enthalpy "eat of Reaction" total change in the bonding energies of a reaction If ΔG is negative (energy released) the reaction is downhill (exothermic). ΔG is positive (energy absorbed) the reaction is uphill (endothermic) 24

28 Thermodynamics The difference in energy is a reflection in the change of the bonds (enthalpy) Bond Dissociation Energy - the energy required to homolytically cleave a bond 25

29 Thermodynamics of Alcohol Substitution R O + Br R Br + O Bonds Broken (energy put in) Bonds Formed (energy released) -O bond 358 kj/mol -Br bond 285 kj/mol -Br bond 366 kj/mol -O bond 465 kj/mol total energy put in 724 kj/mol total energy 750 kj/mol released Δ = = - 26 Thermodynamics only deals with the energy of reactans and products (equilibrium). It says nothing about the path between them. 26

30 Kinetics Kinetics deals with the pathway and describes the rate of reactions - higher energy paths are slower. We can visually map the energy on a reaction potential energy diagram. E A transition state energy barrier to climb ΔG B reaction progress 27

31 Kinetics A multistep reaction path transition state E A slowest step highest energy hill B transition state intermediate ΔG reaction progress 28

32 Some Terms Activation Energy - the energy required to overcome the energy barrier Rate Determining Step - the slowest step in a reaction mechanism - the step with the highest energy barrier Transition State - the structure of molecules at the highest energy point of each energy barrier. The structure of the TS will most closely resemble the nearest intermediate - ammond s Postulate 29

33 Mechanism for Alcohol reaction with X R O + X R X + O Alkyl Alcohol ydrogen alide Alkyl alide Water 30

34 Mechanism for Alcohol reaction with X Step 1 fast R O + Br R O + Br bimolecular reaction Step 2 R O unimolecular reaction slow R + O Step 3 R + Br fast R Br bimolecular reaction 31

35 Step 1 - Protonation of Alcohol 32

36 Step 2 - Dissociation of Water 33

37 Step 3 - Addition of alide 34

38 Overall Reaction Energy Diagram SN1 reaction mechanism Substitution Nucleophilic Unimolecular 35