Chapter 4 Alcohols and Alkyl Halides - PowerPoint PPT Presentation

About This Presentation
Title:

Chapter 4 Alcohols and Alkyl Halides

Description:

Chapter 4 Alcohols and Alkyl Halides – PowerPoint PPT presentation

Number of Views:274
Avg rating:3.0/5.0
Slides: 82
Provided by: FrankC185
Learn more at: http://faculty.sgc.edu
Category:

less

Transcript and Presenter's Notes

Title: Chapter 4 Alcohols and Alkyl Halides


1
Chapter 4Alcohols and Alkyl Halides
2
Functional Groups
  • A functional group is a structural unit in a
    molecule responsible for its characteristic
    physical properties as well as its behavior under
    a particular set of reaction conditions.Alkenes
    and alkynes are examples of functional groups.
  • In this chapter we specifically meet alkylhalides
    and alcohols

3
(No Transcript)
4
Nomenclature
  • IUPAC rules permit the use of two different
    naming conventions. One is functional class
    nomenclature the other is substitutive
    nomenclature.Substitutive nomenclature is
    preferred.Functional class nomenclature is more
    common.

5
Functional Class Nomenclature of Alkyl Halides
  • The alkyl group and the halide are listed
    separately in the name. The alkyl group is the
    longest chain starting at the carbon that has the
    halogen attached. Other alkyl groups are listed
    as substituents.

6
Substitutive Nomenclature of Alkyl Halides
  • Alkyl halides have a halo (fluoro-, chloro-,
    bromo- and iodo-) substituents on an alkane
    chain.
  • The halogen is treated as a substituent.

The carbon chain is numbered from the side
closest the substituent as before.
7
Substitutive Nomenclature of Alkyl Halides
  • Number from side closest to substituent. The
    halogen and alkyl groups have the same priority.
  • In the name list the substituent alphabetically.

2-chloro-5-methylheptane
5-chloro-2-methylheptane
8
IUPAC Nomenclature of Alcohols
  • Functional class names have the alkyl name
    followed by alcohol as a separate word.
  • Substitutive names start with the longest
    contiguous carbon chain that bears the OH group.
    Number from the side closest to the OH group and
    replace the ane of the corresponding alkane with
    ol.
  • List substituents and their locants before the
    parent name.

9
IUPAC Nomenclature of Alcohols
  • The OH is assumed to be attached to C-1 of cyclic
    alcohols.

3-ethylcyclopentanol
4-bromobutan-1-ol
10
Classes of Alcohols and Alkyl Halides
11
Classification of Alkyl Halides
  • Alkyl halides are defined as primary if the
    carbon that the halogen is attached to is
    directly attached to one other carbon. Similarly
    if the carbon that the halogen is attached to is
    directly attached to two carbons then it is a
    secondary alkyl halide. In tertiary alkyl
    halides the carbon with the halogen attached is
    directly attached to three other carbons.

secondary
tertiary
primary
12
Classification of Alcohols
  • Alcohols are defined as primary, secondary or
    tertiary in the same way. If one, two or three
    other carbons are directly attached to the carbon
    that the OH is attached to then the alcohol is
    primary, secondary or tertiary respectively.

secondary
tertiary
primary
13
Bonding in Alcohols and Alkyl Halides
14
Bonding in Alcohols
  • The C-O bond is made by overlap of an sp3 orbital
    on carbon with one on oxygen. The oxygen has two
    non bonding electron pairs.

15
Bonding in Alkyl Halides
  • The halogen is connected to the carbon with a s
    bond.
  • The carbon-halogen bond distances increase in the
    order C-F lt C-Cl lt
    C-Br lt C-I
  • Alkyl halides and alcohols have polar bonds and
    may be polar molecules.

16
Physical Properties of Alcohols and
AlkylHalides Intermolecular Forces
17
Dipole-dipole Attractive Force
  • Molecules with permanent dipoles have a stronger
    dipole-dipole intermolecular interaction than
    alkanes.

Consequently fluoroethane has a higher boiling
point than propane despite being almost the same
size.Dipole-dipole interactions are not enough
to explain the exceptionally high boiling point
of ethanol.
18
Alcohols and Hydrogen Bonding
  • Alcohols have a special type of dipole-dipole
    interaction called hydrogen bonding. The
    partially positive proton of one ? OH group
    interacts with the partially negatively oxygen of
    a second ethanol.

The oxygen is termed the hydrogen bond acceptor
and the OH hydrogen the hydrogen bond donor.
19
Alcohols and Hydrogen Bonding
  • Ball and stick model showing a hydrogen bond
    between two ethanol molecules.

20
Alcohols and Hydrogen Bonding
  • A space-filling model showing the electrostatic
    potential for two hydrogen bonded ethanol
    molecules.

21
Hydrogen Bonding
  • Hydrogen bonds are 15-20 times weaker than
    covalent bonds.
  • Hydrogen bonding in organic compounds involves O
    and N only

Hydrogen bonds are strong enough to impose
structural order on many systems.
22
Boiling Points
Iodine is highly polarizable because the valence
electrons are far from the nucleus. Therefore
the induced dipole-induced dipole attractive
forces dominate.
23
Boiling Points
Increasing the number of halogens (Cl, Br or I)
also increases the induced dipole-induced dipole
attractive forces and therefore also the boiling
point.
24
Boiling Points
Fluorine has very low polarizability and the
boiling points do not increase with increasing
numbers of fluorine atoms.
25
Solubility in Water
Alkyl halides are insoluble in water whereas the
solubilty of alcohols in water is directly
related to the size of the alkyl group the OH is
attached to. Methyl, ethyl, n-propyl, and
isopropyl alcohols are all totally miscible in
water (soluble in all proportions) but only 1 mL
1-octanol dissolves in 2000 mL of water.
Hydrogen bonding between ethanol and water.
26
Density
Alkyl fluorides and chlorides are less dense, and
alkyl bromides and iodides more dense, than
water. Increasing halogenation increases density
so CH2Cl2 is more dense than water.
27
Preparation of Alkyl Halides fromAlcohols and
Hydrogen Halides
28
Preparation of Alkyl Halides
  • Synthesis. The rest of this chapter focusses on
    methods of preparation of alkyl halides.
  • Mechanism.The step-by-step description of how
    reactions take place will be introduced.

29
Preparation of Alkyl Halides
  • Reaction of alcohols with hydrogen halides yields
    alkyl halides

Reactivity of the alcohols is directly related to
the nature of the alcohol
30
Rate of Reaction
  • Tertiary alcohols react fastest at low
    temperature and primary slowest needing higher
    temperatures

31
Reaction of Alcohols with HydrogenHalides The
SN1 Mechanism
32
The Reaction Equation
  • The reaction equation describes the overall
    process from reactants on the left to products
    on the right.

The mechanism will show how this reaction occurs.
33
The Reaction Mechanism Step 1
  • The reaction mechanism is the step-by-step
    pathway describing how the reaction takes place.
  • Step 1 Protonation

The alcohol acts as Brønsted base and is
protonated by the strong acid. Chloride is the
conjugate base of HCl.This is a bimolecular
reaction both reactants change.The
tert-butyloxonium cation is an intermediate.
34
Proton Transfer
  • The change in energy of Step 1 can be plotted on
    a potential energy diagram.

The transition state is not a stable structure
and the bonds are partially formed and partially
broken at this point. The activation energy is
low and the step is exothermic.
35
The Reaction Mechanism Step 2
Step 2 Dissociation.
  • The second step is unimolecular and results in
    the formation of an intermediate carbocation.

36
Carbocation Formation
  • The carbocation intermediate is a relatively
    unstable species and is therefore high energy
    (the central carbon does not have an octet of
    electrons). Overall step 2 is endothermic.

37
Carbocations
  • The central carbon of the carbocation is sp2
    hybridized.The positive charge is in theempty
    p-orbital.
  • Carbocations are electrophilic(electron seeking
    or electronloving) and are Lewis Acids.

38
The Reaction Mechanism Step 3
  • The last step is a Lewis acid-Lewis base
    reaction. The chloride is called a nucleophile
    (nucleus seeker). This is a bimolecular
    reaction.

Step 3 Chloride attaches to the carbocation.
39
Reaction of t-Butyl Cation with Chloride
  • An unstable intermediate reacts to form stable
    products very exothermic. A very favorable
    process so the activation energy is low.

40
Reaction of t-Butyl Cation with Chloride
  • The nucleophile has a nonbonding electron pair in
    a p-orbital that interacts with the empty
    p-orbital of the carbocation to form a s-bond.

41
The Reaction Mechanism
  • The sum of each individual step in the reaction
    mechanism must equal the overall reaction
    equation.
  • The reaction is a substitution reaction in which
    thenucleophile chloride takes the place of the
    OH. Thus, it is known as an SN reaction.The
    slow step in the unimolecular reaction step 2 and
    this is known as the rate determining step. The
    overall reaction cannot go faster than this
    step. Thus, the reaction is a SN1 reaction.

42
Confirming the Mechanism
  • The stereochemistry of a reaction is used to
    probe the formation of a carbocation. For
    example, these two isomeric alcohols should give
    the same carbocation

Therefore they should form exactly the same
product/s as they do a 41 mixture of isomers.
43
Structure, Bonding, and Stability of
Carbocations
44
Stability of Cations
  • Alkyl groups directly attached to the positively
    charged
  • carbon stabilize a carbocation.

Carbocations are defined as primary, secondary or
tertiary depending on how many carbons are
directly attached to the cationic carbon.
45
Stability of Cations
  • The stability of cations can be modeled and the
    spread of the positive charge (blue/violet
    color) seen in the electrostatic potential maps.

Methyl cation has intense positive charge. The
more the charge is delocalized the more stable
the cation is.
46
Stability of Cations
  • A carbocation is stabilized by delocalization
  • of electrons from s-bonds b to the positively
  • charged carbon into the empty p-orbital.
  • The valence bond model shows orbital overlap.
  • MO theory predicts a bonding orbital with 2
    electrons that spans the b s-bond and the
    positive carbon.

47
Effect of Alcohol Structure on Reaction Rate
48
The Alcohol Carbocation Connection
  • The rate determining step is

The rate is only proportional to the
concentration of the alkyloxonium cation.
49
The Alcohol-Carbocation Connection
  • The rate of formation of the carbocation is
    related to the
  • stability of the carbocation formed. The
    transition state
  • is closer in energy to the carbocation so the
    activation
  • energy tracks the stability of the carbocation.

50
Reaction of Methyl and Primary Alcohols with
Hydrogen Halides The SN2 Mechanism
51
Reaction Equation
  • The overall reaction equation looks the same as
    that for
  • tertiary alcohols a substitution reaction.

52
Mechanism Step 1
  • The first step of the reaction is the same.
  • Step 1 Proton transfer.

The second step of the reaction cannot be the
same because the primary carbocation is too
unstable
53
Mechanism Step 2
  • Since primary and methyl carbocations are very
    unstable
  • primary alcohols must react in a different way.
  • Step 2. Nucleophilic displacement.

Step 2 is the slower step and therefore is the
rate determining step. Since this is a
bimolecular reactionit is given the symbol SN2.
54
Other Methods for Converting Alcohols to Alkyl
Halides
55
Reaction of Alcohols with Thionyl Chloride
  • Thionyl chloride is mainly used to transform
    primary and
  • secondary alcohols into alkyl chlorides.

Pyridine is a base used to neutralize the acid
(HCl) formed.
56
The Thionyl Chloride Reaction Mechanism
  • The key step in the reaction is an SN2 type of
    reaction

57
Reaction of Alcohols with Phosphorous Tribromide
  • Phosphorous tribromide (PBr3) reacts with
    alcohols to
  • form alkyl bromides. This is also an SN2 reaction.

58
Halogenation of Alkanes
  • Overall reaction equation
  • RH X2 ? RX HX (XF, Cl, Br or I)
  • For F2 the reaction is explosive
  • For I2 the reaction is endothermic and not
    feasible
  • The reaction is exothermic and useful for Cl2
    and Br2

59
Chlorination of Methane
  • An industrially important reaction carried out at
    high temperature (400 oC). Four products formed
    sequentially.
  • CH4 Cl2 ? CH3Cl HCl
  • CH3Cl Cl2 ? CH2Cl2 HCl
  • CH2Cl2 Cl2 ? CHCl3 HCl
  • CHCl3 Cl2 ? CCl4 HCl
  • The reaction has free radicals as intermediates.

60
Structure and Stability of Free Radicals
61
Free Radicals
  • Free radicals contain unpaired electrons. Some
    common free radicals are

62
Free Radicals
Free radicals with the unpaired electron on a
carbon are defined as primary, secondary or
tertiary depending on how many carbons are
directly attached to the C with the unpaired
electron.
63
Free Radicals
The carbon atom with the unpaired electron is
best described as sp2 hybridized.
64
Stability of Free Radicals
The spin density (shown in yellow) is shared onto
connected alkyl groups thereby stabilizing the
radical .More substituted radicals are more
stable.
65
Stability of Free Radicals
More substituted radicals are more stable.
66
Bond Cleavage
In homolytic bond cleavage each atom retains one
of the bonding electrons. The energy required is
the bond dissociation enthalpy (D). In
heterolytic cleavage the more electronegative
element retains both bonding electrons.
homolytic
heterolytic
67
Bond Dissociation Enthalpies
Comparing the energy required for homolytic bond
cleavageprovides a way to quantify radical
stability. Compare these two reactions
The only difference is the type of radical formed
and the energy released.
68
Bond Dissociation Enthalpies
Potential energy graph of the two homolytic
cleavage reactions. This provides information
on the stability of radicals.
Less energymore stableradical.
69
(No Transcript)
70
Free Radical Chlorination of Methane
We will consider monochlorination.
The mechanism has three parts initiation,
propagation, and termination.
71
Chlorination of Methane Initiation
Step 1 Dissociation.
The chlorine-chlorine bond is broken
homolytically.This bond is the weakest in the
reaction mixture. Heat or light can provide the
necessary energy. Arrows with a single hook
are used to show the movementof a single
electron.
72
Chlorination of Methane Propagation
Step 2 Hydrogen atom abstraction.
A chlorine radical abstracts a hydrogen atom from
a methane molecule forming HCl and a methyl
radical. The methyl radical reacts in the next
step (it propagates the reaction).
73
Chlorination of Methane Propagation
Step 3 Chlorine atom abstraction.
The methyl radical abstracts a chlorine atom from
a chlorine molecule to form chloromethane and
another chlorine radical. The chlorine radical
starts another propagation cycle step 2.
Steps 2 and 3 form a radical chain
reaction. Few radicals are needed to initiate
the chlorination of methane.
74
Chlorination of Methane Termination
When two radicals react they effectively stop the
propagation steps and are therefore known as
chain-termination steps.Examples are
75
Halogenation of Higher Alkanes
Reaction in labs use light (hu) to initiate the
reaction.Cyclobutane yields a single
monochlorination product since abstraction of
any of the 8 H atoms results in the same product
being formed.
In contrast, butane yields a 2872 mixture of
1-chlorobutane to 2-chlorobutane.
76
Selectivity of Halogenation
There are 6 primary hydrogens and 4 secondary
hydrogens in butane. Therefore the expected
ratio is 6040. The 2872 ratio therefore means
that 2-chlorobutaneis preferentially formed.
Why?
77
Selectivity of Halogenation
There must be selectivity when the hydrogen atom
is abstracted. The preferred reaction
is instead of
78
Selectivity of Halogenation
The transition state is lower in energy for
abstraction of a secondary hydrogen because a
secondary radical is more stable
The relative rates of hydrogen abstraction are
79
Selectivity of Chlorination
Based on these experiments the relative rates of
chlorination are determined
80
Selectivity of Bromination
The relative rates of bromination are
Bromination is highly selective favoring tertiary
substitution
81
Chlorination vs Bromination
The hydrogen atom abstraction is exothermic for
chlorination and endothermic and therefore more
selective for bromination
Write a Comment
User Comments (0)
About PowerShow.com