CSUMB
ESSP 311 Organic Chemistry I
Ronald W. Rinehart, Ph.D.
Chapter 4 Alcohols and Alkyl Halides
| A set of PowerPoint slides on
reactions of alkyl halides
in PDF format by Paul R. Young of the University of Illinois at Chicago can be seen at http://www.chem.uic.edu/web1/PDF/CH6.PDF |
| Organic Chemistry
OnLine's page on alkyl halides by Paul R. Young of the University of Illinois at Chicago can be seen at http://www.chem.uic.edu/web1/OCOL-II/WIN/RX.HTM |
| Organic Chemistry
OnLine's page on alcohols by Paul R. Young of the University of Illinois at Chicago can be seen at http://www.chem.uic.edu/web1/OCOL-II/WIN/ALCOHOLS.HTM |
| A set of
PowerPoint slides on radicals
in PDF format by Paul R. Young of the University of Illinois at Chicago can be seen at http://www.chem.uic.edu/web1/PDF/CH10.PDF |
| Carey PowerPoint slides
for chapter 4 from Columbia University [4.1 to 4.3, nomenclature and classes of RX and ROH] http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_01_03.html |
| Carey PowerPoint slides
for chapter 4 from Columbia University [4.4 to 4.5, bonding in and physical properties of RX and ROH] http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_04_05.html |
| Carey PowerPoint slides
for chapter 4 from Columbia University [4.6 to 4.7, acids and bases, mechanism of proton transfer] http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_06_07.html |
| Carey PowerPoint slides
for chapter 4 from Columbia University [4.8 to 4.14, ROH à RX] http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_08_14.html |
| Carey PowerPoint slides
for chapter 4 from Columbia University [4.15 to 4.19, halogenation of alkanes, free-radical reactions] http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_15_19.html |
|
Colby College's
Shockwave tutorial on
Carbocation rearrangements http://www.colby.edu/chemistry/OChem/DEMOS/rearrangements.html |
|
Colby College's
Shockwave tutorial on
nucleophilic substitution http://www.colby.edu/chemistry/OChem/DEMOS/Substitution.html |
| William Reusch of U
Michigan has an excellent Virtual Textbook of Organic Chemistry the section on Chemical Reactivity is at http://www.cem.msu.edu/~reusch/VirtualText/react1.htm#rx1 Reaction Mechanisms at http://www.cem.msu.edu/~reusch/VirtualText/react1.htm#rx4 Reaction Examples at http://www.cem.msu.edu/~reusch/VirtualText/react2.htm#rx5 |
| Alkyl halides by
Gary Trammell and Srinivas Vuppuluri at the University of Illinois at
Springfield has info on structure, preparation and elimination and nucleophilic substitution http://people.uis.edu/gtram1/organic/alkylHalidesmenu.htm |
| Alcohols, ethers, and
thiols by Gary Trammell and Srinivas Vuppuluri at the University of
Illinois at Springfield http://people.uis.edu/gtram1/organic/alcoholsmenu.htm |
Chapter 4: Alcohols and Alkyl Halides
I.
Mechanism in Organic Chemistry.
A. Important Terms and Species
1. Mechanism. [there are
several examples of mechanisms below on this page]
a) The sequence of elementary steps which
describes how a chemical reaction occurs.
b) We will examine many reactions
throughout this course. It is best to learn
and understand these reactions by the
mechanisms in which they occur.
2. Intermediate.
a) A species that exists for some finite
length of time having some stability.
b) It represents an energy minimum in the
course of a reaction.
c) Carbocations and free
radicals are two important examples of intermediates.
3. Transition State.
a) A transient state on the path from one
intermediate to another.
b) It corresponds to an energy maximum.
c) It is drawn in enclosed brackets in
order to indicate its transient character.
4. Free Radical.
a) Species containing an unpaired
electron.
|
|
b) The stability of free radicals
increases from the methyl free radical to the
tertiary free radical:
|
|
|
|
|
|
methyl < |
1o < |
2o < |
3o |
|
least stable |
|
|
most stable |
c) sp2 hybridization.
5. Carbocation.
a) Carbon cations.
b) These are also referred to as carbonium
ions and carbenium ions.
c) As with the free radicals, the
stability of carbocations increases from the
methyl to the tertiary
carbocation.
|
|
|
|
|
|
methyl < |
1o < |
2o < |
3o |
|
least stable |
|
|
most stable |
d) sp2 hybridization.
e) Carbocations and alkyl free
radicals are stabilized by electron-releasing substituents:
alkyl groups release electrons.
i) C-C
sigma bonds are more polarizable than C-H bonds:
electrons in C-C bonds are drawn towards the
positive charge
(or in free radical, the region of low electron
density.)
ii) Delocalization of C-H
sigma bond of a substituent's carbon atom into
the empty p-orbital of a
carbocation (or electron-deficient p-orbital of a free
radical) stabilizes
the species ( this is known as hyperconjugation):
|
|
6. Carbanion.
a) Carbon anions; sp3
hybridization for alkyl carbanions.
7. Exercise. Draw Lewis structures of an ethyl free
radical, carbocation, and carbanion.
|
|
|
|
|
free radical |
carbocation |
carbanion |
|
The Curly Arrows tutorial by Mary Masson at the
University of Aberdeen |
II.
Alkyl Halides
A. Structure of Alkyl Halides.
1. R-X where X = F, Cl, Br, I
2. Classes of Alkyl Halides.
|
a) Primary (1o) Alkyl Halide: |
|
|
b) Secondary (2o) Alkyl Halide: |
|
|
c) Tertiary (3o) Alkyl Halide: |
|
B. Bonding in Alkyl Halides.
1. The C-X bond is usually only sigma.
2. Pi bonds can exist in resonance structures and
intermediates:
|
|
3. C-X bonds are polar.
|
|
C. Nomenclature of Alkyl Halides.
1. Radicofunctional Nomenclature.
a) Alkyl group/halide written as separate
words.
b) Alkyl group named by largest continuous
carbon chain to which the halogen is
attached.
c) Examples:
|
CH3Br |
CH3CH2CH2CH2F |
|
methyl bromide |
n-butyl fluoride |
|
|
|
|
1-propylpentyl iodide |
cyclopentyl chloride |
2. Substitutive
Name.
a) Treat the halogen as a substituent.
b) Use halo- for the name of the
substituent halogen (fluoro-, chloro-, bromo-, iodo-).
c) Halogens and alkyl groups are given
equal priority.
d) Name the following compounds:
|
|
|
|
2-chloro-5-methylheptane |
2-bromo-1,1-dimethylcyclohexane |
|
|
|
|
1-bromo-3-chloro-4-methylhexane |
3,3-dibromo-4-methylheptane |
|
Dave Woodcock has a large number of Chime structures
of halogen-containing molecules at |
D. Physical Properties of Alkyl Halides.
1. Boiling Points. Higher boiling points than
alkanes of similar molecular weight.
a) Boiling points increase with increasing
molecular weight for both the alkyl
group and the halide.
b) Forms of
attraction:
i) London dispersion forces
(induced dipole-induced dipole).
ii) Dipole-induced
dipole.
|
|
iii) Dipole-dipole.
|
|
c) Polarizability of X
i) I > Br > Cl > F
ii) Boiling points:
R-I > R-Br >
R-Cl > R-F
|
|
|
Boiling Points in oC |
|||
|
R |
Formula |
F |
Cl |
Br |
I |
|
Methyl |
CH3X |
-78 |
-24 |
3 |
42 |
|
Ethyl |
CH3CH2X |
-32 |
12 |
38 |
72 |
|
Propyl |
CH3CH2CH2X |
-3 |
47 |
71 |
103 |
|
Isopropyl |
(CH3)2CHX |
-11 |
35 |
59 |
90 |
|
Pentyl |
CH3(CH2)3CH2X |
65 |
108 |
129 |
157 |
|
Hexyl |
CH3(CH2)4CH2X |
92 |
134 |
155 |
180 |
iii) Usually increased # of X's increase the boiling point. Why?
|
CH3Cl |
CH2Cl2 |
CHCl3 |
CCl4 |
|
-24oC |
40oC |
61oC |
77oC |
Answer: Due to an increase of induced dipole-induced dipole interactions.
iv) Increase # of F's can lower boiling point. Why is this trend observed?
|
CH3CH3 |
CH3CH2F |
CH3CHF2 |
CH3CF3 |
CF3CF3 |
|
-89oC |
-32oC |
-25oC |
-47oC |
-78oC |
Answer:
Due to the low polarizability F: a decrease
in induced dipoles. The weak
intermolecular force of attraction causes
fluorinated hydrocarbons (fluorocarbons) to be
low in friction: Teflon
coatings.
2. Solubility of Alkyl Halides.
a) Insoluble in water.
b) Used to dissolve nonpolar organic
compounds. Common solvents: CH2Cl2; CCl4.
3. Density of Alkyl Halides.
a) R-F, R-Cl less dense than water
[true for monohalo cmpds;
polyhalo cmpds can be denser than H2O].
b) R-Br, R-I more dense than water.
c) Solubility characteristics and
difference in density with water: Extractions.
E. Preparation of Alkyl Halides.
1. Halogenation of Alkanes.
|
Halogenation from
the Virtual Textbook of Organic Chemistry by William Reusch at Michigan
State U |
a) R-H + X2
à
R-X + H-X
i) X = Cl, Br
ii) I2 is
unreactive
iii) F2 is
explosive
b) Relative rates of halogenation:
|
X |
3o > |
2o > |
1o |
|
Cl |
5.2 |
3.9 |
1 |
|
Br |
1640 |
82 |
1 |
c) Examples (@25oC) :
CH3CH2CH3
+ Cl2
à
CH3CH2CH2Cl (40%) + CH3CHClCH3
(60%)
CH3CH2CH2CH3
+ Cl2
à
CH3CH2CH2CH2Cl (28%) + CH3CH2CHClCH3
(72%)
CH3CH2CH3 + Br2
à
CH3CH2CH2Br (3%) + CH3CHBrCH3
(97%)
d) Reaction Mechanism. Free Radical.
|
X2 à 2X∙ |
Chain Initiation |
|
X∙
+ R-H à
H-X + R∙ |
Chain Propagation |
|
2R∙
à R-R |
Chain Termination |
e) For the reaction
:
CH4 + Cl2 à
CH3Cl + HCl
Each Cl∙ repeats in the propagation cycle ~5000
times before termination.
f) Why is bromination more selective than
chlorination and why is the order of
reactivity 3o > 2o
> 1o followed?
(See The Hammond Postulate below.)
2. From Alcohols
and Hydrogen Halides.
3. From
Alcohols and Thionyl Chloride.
4. From Alcohols
and Phosphorus Trihalides (PX3, X = Br, Cl)
F. The Hammond Postulate.
1. The Hammond Postulate states that related species
which are similar in energy are also
similar in structure.
2. This general rule tells us something about the
transition states in endothermic and
exothermic reactions.
3. The transition state is always the point of highest
energy on the energy profile.
a) Its structure resembles either the
reactants or the products, whichever
one is higher in energy.
b) In an endothermic reaction the products
are higher in energy, and the transition
state is product-like.
c) In an exothermic reaction, the
reactants are higher in energy and the transition
state is reactant-like.
|
|
|
4. Selectivity in the halogenation of alkanes
can be explained by the Hammond Postulate.
Chlorination:
CH3CH2CH3 + Cl2 à
CH3CH2CH2Cl (40%) + CH3CHClCH3
(60%)
Bromination:
CH3CH2CH3 + Br2 à
CH3CH2CH2Br (3%) + CH3CHBrCH3
(97%)
a) In bromination the transition
state is closer in energy to the free radical intermediate
and therefore also closer to the
free radical in structure:
i) Factors which stabilize
free radicals also stabilize the transition state.
ii) For this reason there is a
large difference in the reactivities for
1o, 2o,
and 3o carbons.
|
|
b) In chlorination the transition
state is closer in energy to the alkane reactant
and therefore also closer to the
alkane in structure:
i)
Factors which stabilize the free radical intermediate are not as important
in the stabilization of the transition
state leading to the
free radical formed (which
in turn leads to the site of halogenation.)
ii) The reactivities of 1o,
2o, and 3o carbons are therefore more similar in
the chlorination of
alkanes resulting in less selectivity.
|
|
III.
Alcohols
A. Structure
1. R-OH
|
a) 1o Alcohols: RCH2OH |
|
|
b) 2o Alcohols: R2CHOH |
|
|
c) 3o Alcohols: R3COH |
|
B. Bonding
1. O bonded to C via a sigma bond.
2. A pi bond between C and O can exist via resonance.
3. The O-H bond is
polar.
|
|
a) Hydrogen bonds can form in
alcohols.
4. C-O-H bond angle ~109.5o (sp3
hybrids).
C. Nomenclature
1. Some simple alcohols have common names which should
be memorized.
|
CH3OH |
CH3CH2OH |
(CH3)2CHOH |
|
methyl alcohol |
ethyl alcohol |
isopropyl alcohol |
|
wood alcohol |
grain alcohol |
rubbing alcohol |
|
Dave Woodcock's Chime structures of alcohols are
at |
2. IUPAC Rules.
a) The parent chain must contain the OH group.
b) The parent chain is named by replacing -ol
for the final -e of the corresponding alkane.
c) The parent chain is numbered so that the OH
group is given the lowest possible number. 3. Name the following alcohols.
|
|
|
|
2-butanol |
1-butanol |
|
|
|
|
3-heptanol |
7-methyl-2-octanol |
|
|
|
|
trans-2-ethylcyclopentanol |
4,4-dimethylcyclohexanol |
|
|
|
|
6,6-dimethyl-3-heptanol |
4-ethyl-1-heptanol |
|
|
5-sec-butyl-6-isopropyl-4-nonanol |
|
Dave Woodcock's page on alcohol nomenclature is
at |
D. Physical Properties of Alcohols
1. Boiling Points.
a) The boiling points of alcohols are higher
than the boiling points of alkanes and alkyl
halides of similar molecular
weights. bpAlc > bpRX and bpRH. Why?
b) Example.
|
cmpd |
CH3CH2CH3 |
CH3CH2F |
CH3CH2OH |
|
MW |
44 |
48 |
46 |
|
bp |
-42oC |
-32oC |
78oC |
Answer: Alcohols can H-bond; no H-bonds can form in RH or RX.
2. Solubility.
a) Low MW alcohols ( < 7 C) are soluble in
water: H-bonds.
b) High MW alcohols are less soluble since
they are more hydrocarbon-like due to a
larger hydrocarbon segment.
c) Alcohols can also dissolve in nonpolar
compounds, for example MeOH in gasoline and paints.
3. Density
a) All liquid alcohols: ~0.8 g/mL.
E. Alcohols as Acids/Bases
1. Brønsted-Lowry
Definition:
a) Acids- Proton Donors
b) Bases- Proton Acceptors
2. Alcohols as acids:
a) ROH + :B
à
BH+ + RO−
Alkoxide Ion (Conjugate Base of ROH)
b) Alcohols: Weaker acids than water
(very similar).
c) H2O + HA
⇌ H3O+ + A−
K = [H3O+][A−]
/ [H2O][HA]
Ka = K[H2O]
= [H3O+][A−]/[HA]
pKa
= -logKa
|
Acid |
Ka |
pKa |
Conjugate Base |
|
HI |
1010 |
-10 |
I− |
|
HCl |
107 |
-7 |
Cl− |
|
HF |
3.5 x 10-4 |
3.5 |
F− |
|
HC2H3O2 |
1.8 x 10-5 |
4.7 |
C2H3O2− |
|
H2O |
1.8 x 10-16 |
15.7 |
OH− |
|
MeOH |
10-16 |
16 |
MeO− |
|
EtOH |
10-16 |
16 |
EtO− |
|
iPrOH |
10-17 |
17 |
RO− |
|
NH3 |
10-36 |
36 |
NH2− |
d) Important points from above:
i) As Ka increases
acid strength increases.
ii) As pKa
decreases acid strength increases.
iii) pKa of
alcohols ~ pKa of water.
iv) Bases which can be used to
effectively deprotonate alcohols must be the
conjugate bases of
acids which have a GREATER pKa than
the corresponding
alcohol.
ROH + NH2−
à
RO− + NH3
e) Alkoxides are important in
organic reactions. Synthesis of alkoxides:
i) Normal preparation:
Reaction of Alcohols and Metals (M = Na, K).
ROH + M
à RO−M+
+ H2
2CH3OH +
2Na à 2CH3O−Na+
+ H2
Order of
reactivity: 3o < 2o < 1o < CH3OH
3o ROH
least reactive: usually use K in place of Na.
ii) Reaction of Alcohols and
NaH.
ROH + NaH
à RO−Na+
+ H2
CH3OH +
NaH à CH3O−Na+
+ H2
f) Alcohols as Bases:
|
|
i) Alkyloxonium ions are important in catalysis of alcohols in reactions.
|
|
F. Reactions of Alcohols. (Preparation of Alkyl Halides.)
1. Alcohols and Hydrogen Halides
a) Overall reaction. ROH + HX
à RX + H2O
b) Examples. Complete the following:
|
|
à |
|
|
|
à |
|
|
|
à |
|
c) Some observations:
i) Reactivity of
Alcohol: 3o > 2o > 1o
< MeOH
ii) Reactivity of HX:
HI > HBr > HCl > HF
iii) Addition of H2SO4
speeds reaction: an acid catalyzed reaction.
iv) A
good method for the
preparation of 3o alkyl chlorides.
d) Mechanism:
i) For 3o, most 2o
ROH: SN1. Substitution Nucleophilic Unimolecular.
|
Animated Chime-like movie of the SN1 mechanism by
Jennifer Muzyka at Centre College |
|
Animated Chime-like movie of the SN1 mechanism
[use Netscape!] from Mol4D at U Nijmegen |
|
Step 1. |
|
|
Step 2. |
|
|
Step 3. |
|
ii) Why does the reaction rate
follow the order 3o > 2o > 1o < MeOH ?
See Figures 4.9 and
4.10 and invoke the Hammond Postulate.
iii) Evidence for carbocation formation: Rearrangements.
|
|
|
|
iv) Show the mechanism for the above reaction. (1,2-hydride shift).
|
Step 1. |
|
|
Step 2. |
|
v) For MeOH, 1o ROH: SN2. Substitution Nucleophilic Bimolecular.
|
Animated Chime-like movie of the SN2
mechanism by Jennifer Muzyka at Centre College |
|
Animated reaction of ethanol with HBr by Richard
C. Banks at Boise State U |
|
QuickTime Movie comments
on factors affecting SN2 substitution by
Brent Iverson at the University of Texas |
|
Animated Jmol [IE OK] and Chime-like [use
Netscape!] movie of the SN2 mechanism
from Mol4D at U Nijmegen |
vi) The reason MeOH reacts
faster than 1o alcohols:
Less hindered to
back-side attack by the nucleophile (X−).
2. Alcohols and Thionyl
Chloride, SOCl2.
a) Overall reaction.
ROH + SOCl2
à
RCl + SO2 + HCl
b) This reaction is mainly used to produce
1o and 2o alkyl chlorides.
c) Examples. Complete the equations:
|
|
|
|
d) Mechanism.
|
|
i) Backside attack by the Cl- on the chlorosulfite ester results in inversion of configuration.
|
|
e) The reaction is run in K2CO3 or pyridine. WHY?
Answer: The base scavenges HCl formed,
preventing acid-catalyzed side reactions.
It also helps pull off H+
from the intermediate.
3. From Alcohols and PX3.
X = Br, Cl, I
a) Overall reaction.
3ROH + PX3
à
3RX + H3PO3
(run in
ether)
b) Examples:
|
|
|
|
|
|
c) Observations:
i) Works best with 1o
and 2o ROH.
ii) Poor yields with 3o
ROH.
iii) SN2 mechanism.
iv) 1o alcohols react by
the slower SN2 process.
PX3 increases the reaction rate by making -OH a better
leaving group.
v) Often the best method to
avoid rearrangement (No Rearrangement).
|
|
|
|
vi) SOCl2 usually
results in better yields than PCl3 in making alkyl chlorides.
d) Mechanism:
|
|
e) Complete the equation and then write the mechanism:
CH3CH2CH2OH + PBr3 à
Many thanks to Rod Oka of
MPC for generously sharing his "Lecture Companion" outline,
reproduced here
in edited form by permission, with
web references and other goodies added by me.
Structures drawn using MDL IsisDraw™ and CS ChemOffice ChemDraw™;
diagrams with MS EXCEL™
updated 9/15/07
