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10. Organohalides

Based on McMurrys Organic Chemistry, 7th

edition

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What Is an Organohalide

An organic compound containing at least one carbon-

halogen bond (C-X)

X (F, Cl, Br, I) replaces H

Can contain many C-X bonds Properties and some uses

Fire-resistant solvents

Refrigerants

Pharmaceuticals and precursors

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Why this Chapter?

Reactions involving organohalides are less

frequently encountered than other organic

compounds, but reactions such as

nucleophilic substitutions/eliminations thatthey undergo will be encountered frequently

Alkyl halide chemistry is model for

mechanistically similar but more complex

reactions

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10.1 Naming Alkyl Halides

Find longest chain, name it as parent chain

(Contains double or triple bond if present)

Number from end nearest any substituent (alkyl or

halogen)

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Naming if Two Halides or Alkyl Are

Equally Distant from Ends of Chain

Begin at the end nearer the substituent whose name

comes first in the alphabet

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10.2 Structure of Alkyl Halides

C-X bond is longer as you go down periodic table

C-X bond is weaker as you go down periodic table

C-X bond is polarized with partial positive charge on

carbon and partial negative charge on halogen

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Preparing Alkyl Halides from Alkanes:

Radical Halogenation Alkane + Cl2 or Br2, heat or light replaces C-H with C-

X but gives mixtures

Hard to control

Via free radical mechanism

It is usually not a good idea to plan a synthesis thatuses this methodmultiple products

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According to Wikipedia, radical is.

Radicals (often referred to as free radicals)

are atoms, molecules, orions with unpaired

electrons or an open shell configuration. Free

radicals may have positive, negative, or zerocharge. With some exceptions, these

unpaired electrons cause radicals to be highly

chemically reactive.

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http://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Unpaired_electronhttp://en.wikipedia.org/wiki/Unpaired_electronhttp://en.wikipedia.org/wiki/Open_shellhttp://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Open_shellhttp://en.wikipedia.org/wiki/Unpaired_electronhttp://en.wikipedia.org/wiki/Unpaired_electronhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Atom

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Radical Chain Mechanism

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Radical Halogenation: Selectivity If there is more than one type of hydrogen in an alkane, reactions

favor replacing the hydrogen at the most highly substituted carbons

H)(1per5%sH')(16

Product))(130% oo

o

H)(2per%5.71sH')(24

Product))(270% oo

o

reactivityrelative1:3.5H)(1per%5

H)(2per%5.71o

o

H)(1per%2.7sH')(19

Product))(165% oo

o

H)(3per35%=sH')(31

Product))(335% oo

o

reactivityrelative1:5H)(1per.2%7H)(3per5%3

o

o

R l ti R ti it

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Relative Reactivity Based on quantitative analysis of reaction products, relative

reactivity is estimated for Cl2: (5:3.5:1 for 3o:2o:1o)

Order parallels stability of radicals

Reaction distinction is more selective with bromine than chlorine(1700:80:1 for 3o:2o:1o)

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Preparing Alkyl Halides from Alkenes Alkyl halide from addition of HCl, HBr, HI to alkenes

to give Markovnikov product (see Alkenes chapter)

Alkyl dihalide from antiaddition of bromine or chlorine

All li B i ti

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Allylic Bromination N-bromosuccinimide (NBS) selectively brominates allylic

positions (Allylic = next to a double bond)

Requires light for activationjust like radical halogenation NBS is just a source of dilute bromine radicals (Br)

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Radical chain mechanism of allylic

bromination.

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All li St bili ti

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Allylic Stabilization Allyl radical is delocalizedradical is shared over 2 carbons

More stable than typical alkyl radical by 40 kJ/mol (9 kcal/mol)

Allylic radical is more stable than tertiary alkyl radical

U f All li B i ti

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Use of Allylic Bromination Allylic bromination with NBS creates an allylic bromide

Reaction of an allylic bromide with base produces a conjugateddiene (by elimination), useful in synthesis of complex molecules

Why this Major Product?

1. Less Hindered Radical

Gives Major Product

2. More substituted alkene

is more stable

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P i Alk l H lid f Al h l

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Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective

Add HCl or HBr gas into ether solution of tertiary alcohol

Primary and secondary alcohols react very slowly and oftenrearrange, so alternative methods are used: SOCl2 or PBr3

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R ti f Alk l H lid G i d R t

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Reactions of Alkyl Halides: Grignard Reagents

Reaction of RX with Mgin ether or THF

Product is RMgX anorganometalliccompound (alkyl-metalbond)

Carbanions (CH3- MgX+)

are very strong bases

Deprotonate water, alcoholsCarboxylic acids, etc

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Organometallic Coupling Reactions

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Organometallic Coupling Reactions Alkyllithium (RLi) forms from RBr and Li metal

RLi reacts with copper iodide to give lithium dialkylcopper(Gilman reagents)

Lithium dialkylcopper reagents react with alkyl halides togive alkanes

Utili ti f O t lli C li i S th i

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Utilization of Organometallic Coupling in Synthesis Coupling of two organometallic molecules produces larger

molecules of defined structure

Aryl and vinyl organometallics also effective

Coupling of lithium dialkylcopper molecules proceeds throughtrialkylcopper intermediate

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Oxidation and Reduction in Organic Chemistry

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Oxidation and Reduction in Organic Chemistry In organic chemistry, we say that oxidation occurs when a

carbon or hydrogen that is connected to a carbon atom in a

structure is replaced by oxygen, nitrogen, or halogen

Not defined as loss of electrons by an atom as in inorganic

chemistry

Oxidation is a reaction that results in loss of electron density

at carbon (as more electronegative atoms replace hydrogen

or carbon)

Organic reduction is the opposite of oxidation

Results in gain of electron density at carbon (replacement

of electronegative atoms by hydrogen or carbon)

Oxidation: breakC-H (or C-C) and form C-O,

C-N, C-X

Reduction: form C-H (or C-C) and breakC-O, C-N, C-X

O id ti d R d ti R ti

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Oxidation and Reduction Reactions

O id i L l

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Oxidation Levels Functional groups are associated with specific oxidation levels

Finding the oxidation level:

Level = (# of C-O,C-N,C-X bonds) (# of C-H bonds)

Ex. 10.12

0-12 = -12 0-6 = -6 2-10 = -8 1-9 = -8

3-0 = 0 1-5 = -4 2-4 = -2