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 ______à Why? ______
---C-X Bond Strength ______à Why? ______
--Dipole Moment (Bond Polarity) ______à Why? ______
Halogens are more electronegative than Carbon. All have substantial dipole moments.
Figure:
10.3 Preparation of Alkyl Halides
Electrophilic Addition
1) Alkene + X2 à
2) Alkene + HX à
X2 = Cl2 or Br2, HX = HCl, HBr, or HI
Free Radical Substitution of Alkanes
See Figure 10.1, p. 320 for Mechanism
Alkane + X2 à
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 --hnà
Butane Ratios:
(only showing monochlorinated products – di, tri, tetra, etc, are possible)
Another Example:
+ Cl2 --hnà
2-Methylpropane Ratios:
Compare equivalent Hydrogen’s 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
------______Stabilityà
The more stable radical forms faster. See Figure 10.2, p. 322.
Bromination is even more selective.
+ Br2 --hnà +
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? (Don’t 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 à
hn, CCl4
Cyclohexene 3-Bromocyclohexene
The presence of hn causes the Br2 to form Br radicals.
Br2 --hvà 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------à
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 à 3-Bromo-1-octene (17%)
hn, CCl4 1-Bromo-2-octene (83%) (53/47- trans/cis)
Mechanism:
Useful Reaction:
NBS à KOH à
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 ------à
Example:
HCl(aq), 25oCà + 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 à
Ether, 35 oC
2-Butanol
SOCl2 à
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 THFà 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 à
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
+ etherà
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, 0oCà
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 à Chloromethane ______Why? ______
Chloromethane 1) Mg, ether/ 2) H3O+à Methane ______Why?______
More Examples on p. 333
See Figure 10.5 on p. 334.
------______Oxidation Level ------à
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, H2NCH2CH2NH2
Problem 10.13, p. 334
Tell whether each of the following reactions is an oxidation, a reduction, or neither. Explain your answers.
15