Transcript 10. Alkyl Halides - University of West Alabama

Chapter 3.
Alcohol and Alkyl Halides
Alkyl Halides
• An organic compound containing at least one carbonhalogen 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
2
3
Naming Alkyl Halides
• Name is based on longest carbon chain
– (Contains double or triple bond if present)
– Number from end nearest any substituent (alkyl or
halogen)
4
5
6
Many Alkyl Halides That Are Widely
Used Have Common Names
•
•
•
•
•
Chloroform
Carbon tetrachloride
Methylene chloride
Methyl iodide
Trichloroethylene
7
Alcohols
• Alcohols contain an OH group connected to a a
saturated C (sp3)
• They are important solvents and synthesis intermediates
• Phenols contain an OH group connected to a carbon in a
benzene ring
• Methanol, CH3OH, called methyl alcohol, is a common
solvent, a fuel additive, produced in large quantities
• Ethanol, CH3CH2OH, called ethyl alcohol, is a solvent,
fuel, beverage
• Phenol, C6H5OH (“phenyl alcohol”) has diverse uses - it
gives its name to the general class of compounds
8
Naming Alcohols
• General classifications of alcohols based on substitution
on C to which OH is attached
• Methyl (C has 3 H’s), Primary (1°) (C has two H’s, one
R), secondary (2°) (C has one H, two R’s), tertiary (3°)
(C has no H, 3 R’s),
9
IUPAC Rules for Naming Alcohols
• Select the longest carbon chain containing the hydroxyl
group, and derive the parent name by replacing the -e
ending of the corresponding alkane with -ol
• Number the chain from the end nearer the hydroxyl group
• Number substituents according to position on chain,
listing the substituents in alphabetical order
10
Many Alcohols Have Common
Names
• These are accepted
by IUPAC
11
Hybridization of Methanol
12
Properties of Alcohols: Hydrogen Bonding
• The structure
around O of the
alcohol or phenol
is similar to that
in water, sp3
hybridized
• Alcohols and
phenolshave
much higher
boiling points
than similar
alkanes and alkyl
halides
Hydrogen bonding in ethanol
Figure 4.4
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.
13
H-Bonding between Ethanol and
Water
ethanol and water
Hydrogen bonding between
Figure 4.5
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.
14
Acids and Bases
The Brønsted-Lowry Definition
• Acid – A proton (H+) donor
• Base – A proton acceptor
Acid
Base
Conjugate
Conjugate
Acid
Base
15
Energy diagram for concerted proton transfer
Figure 4.6
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.
16
Acid and Base Strength
+
-
H + A
HA

-
[H ][ A ]
Ka 
[HA]
pK a   log K a
17
Relative Strengths of Some Common Acids and
Their Conjugate Bases
Acid
Name
pKa
CH3CH2OH
Ethanol
16.00
H2O
Water
15.74
HCN
Hydrocyanic Acid
9.31
CH3CO2H
Acetic Acid
4.76
HF
Hydrofluoric Acid
3.45
HNO3
Nitric Acid
-1.3
HCl
Hydrochloric Acid
-7.0
18
Predicting Acid – Base Reactions
from pKa Values
The proton will always go from the stronger acid
to the stronger base
H O
H O
H C C O
H
+
O H
H
Acetic Acid
pKa = 4.76
H C C O
H
Hydroxide Ion
Acetate Ion
+
H O
H
Water
pKa = 15.74
19
Organic Acids
20
Lewis Acids and Bases
• Lewis Acid – electron-pair acceptor
• Lewis Base – electron-pair donor
• Lewis Acids usually have at least one
empty orbital
• Lewis Bases usually have at least one set
of paired electrons
21
22
Examples of Lewis Acids
HCl
H2O
H
HNO3
H2SO4
OH
O
H H
H C C
H
+
Li
H C C O
O H
H H H
Mg2+
AlCl3
BF3
FeCl3
23
Examples of Lewis Bases
H H
H O H
H C C O
H C
H H H
H
H C
H
H O
H
C
H C
O H
H
O
C
H
H
O C
H
H
C H
H
H
H
O
H C
C C H
H
H
H C O C
H
H
H
H
H C N C
H H H
H
24
Kinds of Organic Reactions
• In general, we look at what occurs and try to learn how it
happens
• Common patterns describe the changes
– Addition reactions – two molecules combine
– Elimination reactions – one molecule splits into two
– Substitution – parts from two molecules exchange
– Rearrangement reactions – a molecule undergoes
changes in the way its atoms are connected
25
How Organic Reactions Occur:
Mechanisms
• In a clock the hands move but the mechanism
behind the face is what causes the movement
• In an organic reaction, we see the
transformation that has occurred. The
mechanism describes the steps behind the
changes that we can observe
• Reactions occur in defined steps that lead from
reactant to product
26
Steps in Mechanisms
• We classify the types of steps in a sequence
• A step involves either the formation or breaking of a
covalent bond
• Steps can occur in individually or in combination with
other steps
• When several steps occur at the same time they are said
to be concerted
27
Types of Steps in Reaction
Mechanisms
• Formation of a covalent bond
– Homogenic or heterogenic
• Breaking of a covalent bond
– Homogenic or heterogenic
• Oxidation of a functional group
• Reduction of a functional group
28
Breaking of Covalent Bonds
Homolytic Cleavage
• Each product gets one electron from the bond
• Not common in organic chemistry
Heterolytic Cleavage
• Both electrons from the bond that is broken become
associated with one resulting fragment
• A common pattern in reaction mechanisms
29
Formation of a Bond
Homogenic
• One electron comes from each fragment
• No electronic charges are involved
• Not common in organic chemistry
Heterogenic
• One fragment supplies two electrons
• One fragment supplies no electrons
• Combination can involve electronic charges
• Common in organic chemistry
30
Indicating Steps in Mechanisms
• Curved arrows indicate breaking and
forming of bonds
• Arrowheads with a “half” head (“fishhook”) indicate homolytic and
homogenic steps (called ‘radical
processes’)
• Arrowheads with a complete head
indicate heterolytic and heterogenic
steps (called ‘polar processes’)
31
5.6 Using Curved Arrows in Polar
Reaction Mechanisms
• Curved arrows are a way to keep track of changes in
bonding in polar reaction
• The arrows track “electron movement”
• Electrons always move in pairs
• Charges change during the reaction
• One curved arrow corresponds to one step in a reaction
mechanism
32
5.4 Polar Reactions and How They
Occur
• Molecules can contain local unsymmetrical electron
distributions due to differences in electronegativities
• This causes a partial negative charge on an atom and a
compensating partial positive charge on an adjacent
atom
• The more electronegative atom has the greater electron
density
33
Electronegativity of Some Common
Elements
• The relative electronegativity is indicated
• Higher numbers indicate greater electronegativity
• Carbon bonded to a more electronegative element has a
partial positive charge (+)
34
Polarizability
• Polarization is a change in electron distribution as a
response to change in electronic nature of the
surroundings
• Polarizability is the tendency to undergo polarization
• Polar reactions occur between regions of high electron
density and regions of low electron density
35
Generalized Polar Reactions
• An electrophile, an electron-poor species, combines
with a nucleophile, an electron-rich species
• An electrophile is a Lewis acid
• A nucleophile is a Lewis base
• The combination is indicate with a curved arrow from
nucleophile to electrophile
36
10.7 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
often rearrange, so alternative methods are used
37
Energy diagram for formation of tert-butyl chloride
from tert-butyl alcohol and hydrogen chloride
Figure 4.7
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.
38
5.10 Describing a Reaction:
Intermediates
• If a reaction occurs in more than one step, it must
involve species that are neither the reactant nor the final
product
• These are called reaction intermediates or simply
“intermediates”
• Each step has its own free energy of activation
• The complete diagram for the reaction shows the free
energy changes associated with an intermediate
39
Structure of methyl cation
Figure 4.8
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.
40
Combination of a carbocation and a halide anion
Figure 4.11
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.
41