CHM 2211C

6th edition Notes

 

Chapter 10

Alkyl Halides

 

By

Dr. Andrea Wallace

Coastal Georgia Community College

 

Edited by

John T. Taylor

Florida Community College at Jacksonville
Chapter 10: Alkyl Halides

 

Alkyl halides occur widely in nature and have many uses in industrial processes.

 

Uses of Alkyl Halides see p. 316 solvents, anesthetic, refrigerant, fumigant

Epibatidine (p. 317) is found on the skin of an Ecuadorian frog and is 200 x more potent than morphine in blocking pain.

 

10.1 Naming Alkyl Halides

 

Rule 1) Find the parent chain longest continuous chain (that contains a double or triple bond if one is present).

 

Rule 2) Assign lowest numbers to the branches alkyl and halo.

a)      Apply prefixes di, tri, tetra

b)      Alphabetize

 

Rule 3) If the parent chain can be properly numbered from either end by Rule 2, assign them alphabetically.

 

Common names: Alkyl group + Halide

Example: Iodomethane (IUPAC) or methyl iodide (common)

Example: 2-Chlorpropane (IUPAC) or isopropyl chloride (common)

 

Problem 10.1, p. 318

Give the IUPAC names of the following alkyl halides.

 

a)

 

 

 

d)

 

 

 

 

f)

 

 

 

Problem 10.2, p. 318

Draw the structures corresponding to the following IUPAC names:

d)    1,1-Dibromo-4-isopropylcyclohexane

 

 

 

 

 

10.2 Structures of Alkyl Halides

 

Table 10.1, p. 319

 

CH3F CH3Cl CH3Br CH3I

 

----C-X Bond Length ________________ Why? ______________________________

 

---C-X Bond Strength _______________ Why? ______________________________

 

--Dipole Moment (Bond Polarity) ___________ Why? _________________________

 

Halogens are more electronegative than Carbon. All have substantial dipole moments.

 

Figure:

 

 

 

 

 

 

10.3 Preparation of Alkyl Halides

 

Electrophilic Addition

 

1) Alkene + X2

 

 

2) Alkene + HX

 

 

X2 = Cl2 or Br2, HX = HCl, HBr, or HI

 

 

Free Radical Substitution of Alkanes

See Figure 10.1, p. 320 for Mechanism

 

Alkane + X2

 

 

 

 

 

 

 

 

 

10.4 Radical Halogenation of Alkanes

 

Why is this not necessarily the best choice?

 

 

What are the possible products of CH4 + Cl2?

 

 

 

 

There are even more products possible when more than one type of Hydrogen is present.

 

Example:

 

 

 

+ Cl2 --hn

 

Butane Ratios:

 

 

(only showing monochlorinated products di, tri, tetra, etc, are possible)

 

Another Example:

 

 

 

+ Cl2 --hn

 

2-Methylpropane Ratios:

 

 

Compare equivalent Hydrogens and Ratios:

 

Butane 2-Methylpropane

 

 

 

 

____eq. Primary ___ eq. Primary

____eq. Secondary ___ eq. Tertiary

 

 

 

 

 

Primary accounts for ___% of pdt. Primary accounts for ___% of pdt.

Secondary accounts for ___% of pdt. Tertiary accounts for ___% of pdt.

 

 

Secondary is _____ times more likely. Tertiary is _____ times more likely.

See Reactivity Figure on p. 321.

 

Primary < Secondary < Tertiary

 

 

 

This shows relative reactivity towards Chlorination.

 

Why? Table 5.3, p. 154

 

Energy needed to break bond

 

Primary

 

Secondary

 

Tertiary

 

 

Radical Stability

 

 

 

 

-------_____________________ Stability

 

The more stable radical forms faster. See Figure 10.2, p. 322.

 

Bromination is even more selective.

 

+ Br2 --hn +

 

2-methylpropane 2-bromo-2- 1-bromo-2-

methylpropane methylpropane

> 99% < 1%

 

Why?

Hammond Postulate the transition state most closely resembles the species (reactant or product ) to which it is closest in energy.

 

DH = -50 kJ for X = Cl

DH = +13 kJ for X = Br

 

The bromine reaction is more ____________ and thus more product-like (more similar to the radical). The reaction shows selectivity that reflects the stability of the radical.

 

 

 

Problem 10.3, p. 323

Draw and name all monochloro products you would expect to obtain from radical chlorination of 2-methylpentane. Which, if any, are chiral?

 

 

 

 

 

 

 

 

 

Problem 10.4, p. 323

Taking the reactivities of 1o, 2o, and 3o hydrogen atoms into account, what product(s) would you expect to obtain from monochlorination of 2-methylbutane? What would the approximate percentage of each product be? (Dont forget to take into account the number of each type of hydrogen.)

 

 

 

 

 

 

 

 

 

 

 

 

 

10.5       Allylic Bromination of Alkenes

 

Find the allylic positions on cyclohexene.

 

 

 

 

 

Reaction: Allyic Bromination of an Alkene

 

 

 

NBS

hn, CCl4

 

Cyclohexene 3-Bromocyclohexene

The presence of hn causes the Br2 to form Br radicals.

 

Br2 --hv 2 Br .

 

 

Mechanism:

 

 

 

 

 

 

 

 

 

 

 

What type of intermediate is formed? ________________________________

What step of the mechanism is shown above? __________________________

What step of the mechanism is occurring when the Br radical forms (shown above the mechanism)? ________________________________

What contributes to the stability of the allylic radical? _____________________

 

Why does bromination occur exclusively at the allylic position?

 

Compare bond dissociation energies at the other positions alkyl, allylic, and vinylic.

 

 

 

 

 

 

The bond dissociation energy is less because the radical formed is more stable.

 

 

 

 

 

--------------------------_________________ Radical Stability----------

 

10.6       Stability of the Allyl Radical: Resonance Revisited

 

Why are allylic radicals so stable?

Draw the resonance structures for allyl radical.

 

 

Delocalization of electrons via resonance gives increased stability.

The greater the # of resonance structures, the greater the stability of the molecule.

 

Example: Propyl radical

Electron is localized on Carbon it is _________ stable.

 

 

Bromination of unsymmetrical alkenes yields an unequal mixture of products.

Reaction at less hindered end is more stable.

 

 

1-octene NBS 3-Bromo-1-octene (17%)

hn, CCl4 1-Bromo-2-octene (83%) (53/47- trans/cis)

 

Mechanism:

 

 

 

 

 

 

 

 

 

 

 

 

 

Useful Reaction:

 

NBS KOH

hn, CCl4

 

Cyclohexene 3-Bromocyclohexene 1,3-Cyclohexadiene

 

 

What type of reaction occurs when KOH is added? ____________________

 

Problem 10.5, p. 327

Draw three resonance forms for the cyclohexadienyl radical.

 

 

 

 

 

Problem 10.6, p. 327

The major product of the reaction of methylenecyclohexane with N-bromosuccinimide is 1-(bromomethyl)cyclohexene. Explain.

 

 

 

 

 

 

 

 

Problem 10.7 b., p. 327

What products would you expect from reaction of the following alkenes with NBS? If more than one product is formed, show the structures of all.

 


10.7       Preparing Alkyl Halides from Alcohols

 

Most general method of preparation of alkyl halides.

 

Reaction:

 

 

 

 

Alcohol Hydrohalic acid Alkyl Halide Water

 

where X = Cl, Br, or I

Works best for 3o Alcohols.

1o and 2o alcohols react very slowly and require very high temperatures generally not practical.

 

Reactivity of Alcohols with Hydrohalic acids

 

 

 

 

Methyl Primary Secondary Tertiary

 

------_______________ reactivity -------

 

Example:

 

 

HCl(aq), 25oC + H2O

 

 

t-butyl alcohol t-butyl chloride

 

 

Primary and Secondary alcohols are best converted to alkyl halides by reaction with Thionyl Chloride (SOCl2) or Phosphorus Tribromide (PBr3).

 

Examples:

 

 

PBr3

Ether, 35 oC

 

 

2-Butanol

 

 

 

 

SOCl2

Pyridine

 

 

 

2-Butanol

 

 

Why does this reaction work better for primary and secondary alcohols? These reagents are less acidic and less likely to cause acid catalyzed rearrangements.

 

(Mechanisms are covered in Chapter 11.)

 

Problem 10.8, p. 369

How would you prepare the following alkyl halides from the appropriate alcohols?

a)      2-chloro-2-methylpropane

 

 

 

 

 

b)      1-bromo-5-methylhexane

 

 

 

 

 

 

 

10.8       Grignard Reagents

 

Grignard reagents are organometallic reagents, RMgX

 

 

Preparation of Grignard Reagents:

 

R-X + Mg Ether or THF R-Mg-X

Alkyl halide Grignard Reagent

 

 

where R = 1o, 2o, or 3o alkyl, aryl, or alkenyl (all work equally well best to use THF with aryl and alkenyl)

X = Cl, Br, or I (Cl is less reactive than Br or I, organofluorides rarely react with Mg)

 

Examples:

 

 

 

 

 

Bromobenzene Phenylmagnesium bromide

 

 

 

 

 

 

 

2-chlorobutane sec-butylmagnesium chloride

 

 

 

 

 

Polarity of C-Mg bond:

 

 

 

 

 

 

Think of the C as partial negative or even negative like a carbanion. These species do act like bases and react with acids (proton donors) such as H2O, ROH, RCOOH, and RNH2 to yield hydrocarbons.

 

Example:

 

 

 

1-bromobutane butane

 

 

Problem 10.10, p. 330

How might you replace a halogen substituent with a deuterium atom if you wanted to prepare a deuterated compound?

 

 

 

10.9       Organometallic Coupling Reactions

 

Preparation of Organometallic Reagents

 

 

2 Li

pentane

 

 

1-bromobutane Alkyllithium Lithium Bromide

(Butyllithium)

 

Alkyllithiums are basic and act as nucleophiles. They are similar to RMgX (Grignard Reagents).

 

One of the most valuable reactions of alkyllithiums is the preparation of diorganocopper compounds or Gilman reagents.

 

Preparation of Gilman Reagents

 

 

 

 

+ ether

 

 

Methyllithium Copper(I) Iodide Lithium Dimethylcopper Lithium Iodide

(Gilman Reagent)

 

 

Reaction of Gilman Reagent with Alkyl Halides (Cl, Br, or I) to Produce Alkanes

 

 

 

 

Ether, 0oC

 

 

Lithium Dimethylcopper Ethyl Iodide Propane

(Gilman Reagent)

 

 

This organometallic coupling reaction is very versatile. It also works with vinylic halides and aryl halides (not just alkyl halides). See p. 331.

 

Example:

Problem 10.11, p. 332

How would you prepare the following compounds using an organocopper coupling reaction? More than one step is required in each case.

 

a)      3-methylcylohexene from cyclohexene

 

 

 

 

 

 

 

 

 

 

c)      Decane from 1-pentene

 

 

 

 

 

 

 

 

 

 

 

10.10  Oxidation and Reduction in Organic Chemistry

 

Inorganic Definitions:

 

Oxidation - ____________________ of electrons

Reduction - ____________________ of electrons

 

Still true in organic, but the definition is different.

 

Oxidation is a gain of _______________________onto C and/or a loss of __________ onto C.

 

Reduction is a gain of _______________________onto C and/or a loss of __________ onto C.

 

Examples on p. 333

 

Methane + Cl2 Chloromethane _________________ Why? __________________

 

Chloromethane 1) Mg, ether/ 2) H3O+ Methane _______________________ Why?____________________________

 

More Examples on p. 333

 

See Figure 10.5 on p. 334.

 

 

 

 

 

 

--------------------------_________________ Oxidation Level -----------------

 

 

 

 

 

 

Problem 10.12, p. 334

Rank each of the following series of compounds in order of increasing oxidation level:

(Strategy: Compounds that have the same number of carbon atoms can be compared by adding the number of C-O, C-N, and C-X bonds in each and then subtracting the number of C-H bonds. The larger the resultant value, the higher the oxidation level.)

 

a)

 

 

 

 

 

 

 

 

b)      CH3CN, CH3CH2NH2, H2NCH2 CH2NH2

 

 

 

 

Problem 10.13, p. 334

Tell whether each of the following reactions is an oxidation, a reduction, or neither. Explain your answers.