page minto - lectures 7-8 1 chapter 4 - “alcohols and alkyl halides” 1)functional groups and...

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Minto - Lectures 7-8

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Chapter 4 - “Alcohols and Alkyl Halides”

1) Functional groups and nomenclature2) Role of intermolecular forces on properties3) Synthesis of alkyl halides from alcohols

4) Two reaction mechanisms: SN1 and SN2cation stability and the Hammond Postulate

5) Additional ways to prepare RX from ROH6) Halogenation of alkanes: reactivity-selectivity

principle, radical stability and chain reactions

All sections are covered.

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Functional Groups and Priorities

Nomenclature assembles names like trees: start with a parent structure (e.g., an alkane), then add prefixes and a functional group (FG) suffix.

FGs are the centers of reactivity within molecules and have predictable chemical behavior.

In naming, FGs are usually higher priority than a substituent. In more complicated names, FGs can be indicated by a prefix.

IUPAC naming of acyclic compounds

Double and triple bonds are incorporated into the parent chain, as much as possible, resulting in a modified hydrocarbon name. Beyond their presence in the parent chain, they are numbered in a prioritized fashion with any other FGs that are present.

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FG Priority List

High

CarboxylateCarboxylic acidAnhydrideEsterAcid halideAmideNitrileAldehydeKetoneAlcohol (phenol)AmineEtherAlkeneAlkyneAlkyl halide, phenyl, alkyl

Low

hsinman

Memorize Me :D

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IUPAC Nomenclature of Alkyl HalidesCH3Cl CH3CH2F

CH3CHI

CH3

CH3CH2CHBr

CH3chloromethane fluoroethane2-iodopropane 2-bromobutane

1. In the formal IUPAC system, alkyl halides are named as substituted alkanes: F, fluoro; Cl, chloro; Br, bromo; I, iodo. In common naming, they are named as halides (e.g., methyl chloride).

2. Halogens are treated as substituents of equal priority to alkyl groups.

Cl

Br

I

4-chloro-3,3-dimethyl-6-(1-methylethyl)nonaneor4-chloro-6-isopropyl-3,3-dimethylnonane

4-bromo-2-iodo-1-methylcyclohexane

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Classification of Alkyl Halides

Br Br Br

n-butyl bromide (common)or 1-bromobutane (IUPAC)

sec-butyl bromide (common)or 2-bromobutane (IUPAC)

tert-butyl bromide (common)Or 2-bromo-2-methylpropane (IUPAC)

1° 2° 3°

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Nomenclature of AlcoholsIn an alcohol, the OH is a high-priority functional group.

IUPAC NAME = substituent prefixes+parent+functional group suffix(es). For alcohols, alkane suffix “e” changes to “ol”.

1. Determine the parent hydrocarbon containing the functional group. On cyclic compounds, OH receives number “1”; only show the “1” if multiple OH groups are present.2. If necessary, the suffix is preceded by a positional identifier.

**

**both o.k. as formal names

**

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1°

OHOH

2°3°

Alcohols can be categorized as primary (1°), secondary (2°),and tertiary (3°) by the number of carbons directly attached to the hydroxyl-bearing carbon.

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NH2

NH

N

1-butanamine or butan-1-amine

A primary (1°) amine, one C attached to N

Classification of amines is different than ROH or RX.

N-ethyl-3-heptanamine

A secondary (2°) amine, two C attached to N

2-ethyl-N-methyl-N-propylcyclohexanamine

A tertiary (3°) amine, three C attached to N

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Hybridization and Bonding in ROH

= 1.7 D= 1.7 D = 1.9 D= 1.9 D

MeOH MeCl

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Recall:A hydrogen bond is a very strong dipole–dipole interaction: ONLY occurs when N-H, O-H, and F-H bonds (H-bond donors)interact with N, O, and F atoms (H-bond acceptors).

Solubility of hydrogen-bondable molecules in water depends upontheir proportion of H-bond donor/acceptor atoms.

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44 48 46

-42 -32 +78

0 1.9 1.7

CHCH33CHCH22CHCH33 CHCH33CHCH22FF CHCH33CHCH22OHOH

MolecularMolecularweightweight

BoilingBoilingpoint, °Cpoint, °C

DipoleDipolemoment, Dmoment, D

Effect of Structure on Boiling Point

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Polarizability

Distortion of electron clouds is important to van der Waals attractive interactions (induced dipole-induced dipole).

Increasing the atomic radii within a group raises the boiling point.

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Example: Boiling point increases with increasing number of halogens

CH3Cl

-24°C

CH2Cl2

40°C

CHCl3

61°C

CCl4

77°C

CompoundCompound Boiling PointBoiling Point

Even though CClEven though CCl4 4 is the only compound in this list without is the only compound in this list without

a dipole moment, it has the highest boiling point.a dipole moment, it has the highest boiling point.

Induced dipole-induced dipole forces are greatest in CClInduced dipole-induced dipole forces are greatest in CCl44

because it has the greatest number of Cl atoms. Cl is more because it has the greatest number of Cl atoms. Cl is more polarizable than H. polarizable than H.

Zero dipole moment

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???

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Reaction MechanismsWe will dissect the reactions of alcohols with HX to make alkylhalides to learn about reaction mechanisms. There are two main reaction types SN1 and SN2. We will define these names in a few minutes.

Example: (CH3)3COH + HCl ------> (CH3)3CCl + HOH

This reaction is a substitution reaction, but the net reaction doesnot give the details of the reaction mechanism, or path.

Ten new terms: Elementary step IntermediateTransition state Oxonium ionActivation barrier CarbocationConcerted reaction NucleophileMolecularity Electrophile

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Reaction Pathway

Energy Barrier

Intermediate

2nd intermediate

Products

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tert-butanol + HCl, 1st elementary step

pKa -3.9pKa -2

Intermediates

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tert-butanol + HCl, 2nd elementary step

Hammond Postulate: If the structure of a transition state is similar in energy to the species immediately preceding or following it, the transition state structure will most resemble that species.

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tert-butanol + HCl, 3rd elementary step

Alkyl Chloride

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20Potential Energy Diagram of SN1 Reaction of an Alcohol with HX

Rate = k[alkyloxonium ion]

Mechanisms can be Mechanisms can be supportedsupported or or disproveddisproved, but , but cannotcannot be be provenproven correct. correct.

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