organic chemistry – the functional group...

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Organic Chemistry – The Functional Group Approach

alkane(no F.G.)

non-polar (grease, fats)

tetrahedral

OH

alcohol

polar (water soluble)

tetrahedral

Br

halide

non-polar (water insoluble)

tetrahedral

alkene


trigonal

alkyne


linear

aromatic


flat

aldehyde/ketone


trigonal

imine


trigonal

O NH

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OCH3

carboxylic ester

polar (water-solube)

trigonal

NH2

carboxylic amide


trigonal

Cl

acyl halide

non-polar (reacts w/water)

trigonal

O

acid anhydride

non-polar (reacts w/water)

trigonal

O O O O O

hydrate


tetrahedral

acetal


tetrahedral

amine


tetrahedral

OH

carboxylic acid


trigonal

NH2 OHO OH H3CO OCH3

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alkane(no F.G.)


tetrahedral

OH

alcohol


tetrahedral

Br

halide


tetrahedral

alkene


trigonal

alkyne


linear

aromatic


flat

aldehyde/ketone


trigonal

imine


trigonal

O NH

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alkane(no F.G.)


tetrahedral

OH

alcohol


tetrahedral

Br

halide


tetrahedral

alkene


trigonal

alkyne


linear

aromatic


flat

aldehyde/ketone


trigonal

imine


trigonal

O NH

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Carey Chapter 4 – Alcohols and Alkyl Halides

Figure 4.2 – Electron density maps of CH3OH and CH3Cl

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Alcohols and Halogens in Medicine and Nature

ChloramphenicolAcetaminophen

O2NHN

O

OH OH

Cl

Cl

Valium

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4.2 IUPAC Nomenclature of Alkyl Halides

• Functional class nomenclature

pentyl chloride cyclohexyl bromide 1‐methylethyl iodide

• Substitutive nomenclature

2‐bromopentane 3‐iodopropane 2‐chloro‐5‐methylheptane

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4.3 IUPAC Nomenclature for Alcohols

1‐pentanolcyclohexanol

2‐propanol

2‐pentanol 1‐methyl cyclohexanol 5‐methyl‐2‐heptanol

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4.4 Classes of Alcohols and Alkyl Halides

Cl OHBr

OH ICl

BrCH3

(CH3)3COHCH2CH3

Cl

Primary (1o)

Secondary (2o)

Tertiary (3o)

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4.5 Bonding in Alcohols and Alkyl Halides

Figure 4.1

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4.5 Bonding in Alcohols and Alkyl Halides

Figure 4.2 – Electron density maps of CH3OH and CH3Cl

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4.6 Physical Properties – Intermolecular Forces

CH3CH2CH3 CH3CH2F CH3CH2OH

propane fluoroethane ethanol

b.p. ‐42 oC ‐32 oC 78 oC

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Figure 4.4

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Figure 4.4

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4.6 Physical Properties – Water Solubility of Alcohols

Alkyl halides are generally insoluble in water (useful in lab)

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4.6 Physical Properties – Water Solubility of Alcohols

Solubility is a balance between polar and non‐polar characteristics

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4.6 Physical Properties – Water Insolubility

Biochemistry involves a delicate balance of “like dissolves like”

Cholesterol – non‐polar alcohol Limited solubility in water Precipitates when to concentrated Results in gallstones

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4.7 Preparation of Alkyl Halides from Alcohols and H-X

R OH + H X R X + H O H

alcohol hydrogen halide alkyl halide water

Lab Conditions

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4.8 Mechanism of Alkyl Halide Formation

Mechanism – a description of how bonds are formed and/or broken when

converting starting materials (left hand side) to products (right hand side)

Usually involves solvents and reagents, sometimes catalysts

Curved arrows are used to describe the chemical changes

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4.8 Reaction of a Tertiary Alcohol with H-Cl

Look for chemical changes – which bonds are formed or broken?

learn the outcome of reaction in order to get going quickly

recognize the nature of the organic substrate (1o, 2o, 3o?)

be aware of the reaction conditions (acidic, basic, neutral?)

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4.8 Reaction of a Tertiary Alcohol with H-Cl

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4.8 Energetic description of mechanism - Step 1 : protonation

Figure 4.6

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4.8 Energetic description of mechanism - Step 2 : carbocation

Figure 4.7

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4.8 Energetic description of mechanism - Step 3 : trap cation

Figure 4.9

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4.9 Full mechanism “pushing” curved arrows

H3C

CH3C

H3C

O H

H Cl H3C

CH3C

H3C

Cl

H3C

CH3C

H3C

O H

H

C

CH3

H3C CH3Cl

Cl

(+ H2O)

H Cl

(- H2O)

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4.9 Full SN1 mechanism showing energy changes

Figure 4.11

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4.10 Carbocation structure and stability

Figure 4.8

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Hyperconjugation – the donation of electron densityfrom adjacent single bonds

4.10 Carbocation structure and stability

Figure 4.15

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4.10 Relative carbocation stability

Figure 4.12

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4.11 Relative rates of reaction of R3COH with HX

Related to the stability of the intermediate carbocation:

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4.11 Relative rates of reaction of R3COH with HX

Rate‐determining step involves formation of carbocation

Figure 4.16

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4.12 Reaction of methyl- and 1o alcohols with HX – SN2

Same bonds are formed and broken as in 3o case, however;

CH3 and 1o carbon cannot generate a stabilized carbocation

kinetic studies show the rate‐determining step is bimolecular

sequence of bond‐forming/breaking events must be different

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4.12 Reaction of methyl- and 1o alcohols with HX – SN2

Alternative pathway for alcohols that cannot form a good carbocation

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4.12 Geometry changes during SN2

http://www.bluffton.edu/~bergerd/classes/cem221/sn‐e/SN2.gif

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4.12 Energy profile for SN2 reaction

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4.13 Other methods for converting ROH to RX

OH PBr3 BrSOCl2Cl

Convenient way to halogenate a 1o or 2o alcohol

Avoids use of strong acids such as HCl or HBr

Via SN2 mechanism at 1o and CH3 groups

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4.14 Free Radical Halogenation of Alkanes

heterolytic

homolytic

Possible modes of bond cleavage

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4.15 Free Radical Chlorination of Methane

CH4 + Cl2

CH3Cl + Cl2(~400oC)

CH2Cl2 + Cl2

CHCl3 + Cl2

(~400oC)

(~400oC)

(~400oC)

CH3Cl + HCl

CH2Cl2 + HCl

CHCl3 + HCl

CCl4 + HCl

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4.16 Structure and stability of Free Radicals

Figure 4.17 – Bonding models for methyl radical

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4.16 Structure and stability of Free Radicals

Free radical stability mirrors that of carbocations

Hyperconjugation is the main factor in stability

Experimental evidence that radicals are flat (sp2)

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4.16 Bond Dissociation Energies (BDE)

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4.16 Bond Dissociation Energies (BDE)

104 58 83.5 103

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4.17 Mechanism for Free Radical Chlorination of Methane

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4.18 Free Radical Halogenation of Higher Alkanes

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Radical abstraction of H is selective since the stability of the ensuing radical is reflected in the transition state achieved during abstraction.

Cl H CH2CH2CH2CH3

Cl H CHCH2CH3

CH3

Lower energy radical, formed faster

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Figure 4.16

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4.18 Bromine radical is more selective than chlorine radical

Consider propagation steps – endothermic with Br∙, exothermic with Cl∙

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4.18 Bromine radical is more selective than chlorine radical

Bromination – late TS looks a lot like radical

Chlorination – early TS looks less like radical

organic chemistry – the functional group...

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