Transcript Chapter 5. An Overview of Organic Reactions

CH 5: AN OVERVIEW OF
ORGANIC REACTIONS
Vanessa N. Prasad-Permaul
CHM 1046
Valencia College
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Why this chapter?
 To understand organic and/or biochemistry, it
is necessary to know:
-What occurs
-Why and how chemical reactions take place
We will see how a reaction can be described
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HYDROCARBONS
SATURATED HYDROCARBONS: hydrocarbons that contain
only single bonds between carbon atoms
UNSATURATED HYDROCARBONS: hydrocarbons that contain
double or triple bonds between carbon atoms
AROMTAIC HYDROCARBONS: hydrocarbons that contain
benzene rings or similar features
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The Bonding of Carbon
Alkane
C
Single Bond
C
Double Bond
C
Two Double Bond
C
Triple Bond
Alkene
Alkyne
Alkanes
 Alkanes: Compounds with C-C single bonds and




C-H bonds only (no functional groups)
Connecting carbons can lead to large or small
molecules
The formula for an alkane with no rings in it
must be CnH2n+2 where the number of C’s is n
Alkanes are saturated with hydrogen (no more
can be added
They are also called aliphatic compounds
Structures of Alkanes
Branched-Chain Alkanes
Isomers: compounds with the same molecular formula
but different structural formulas
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Branched-Chain Alkanes
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EXAMPLE 23.1
EXERCISE 23.1
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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
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SUBSTITUTION REACTIONS OF ALKANES
Substitution
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ALKENES
GEOMETRIC ISOMERS: isomers in which the atoms are joined to
one another in the way but differ because some atoms occupy
different relative positions in space
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EXAMPLE 23.2
EXERCISE 23.2
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REARRANGEMENT OF ALKENES
Rearrangement
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ADDITION OF ALKENES
Addition
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MARKOWNIKOFF’S RULE:
"when an unsymmetrical alkene reacts with a hydrogen halide to give an
alkyl halide, the hydrogen adds to the carbon that has the greater number
of hydrogen substituents yielding the major product"
2-bromopropane
1-bromopropane
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EXAMPLE 23.3
EXERCISE 23.3
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ALKYNES
Alkynes are hydrocarbons that have a triple bond between
two carbon atoms, with the formula CnH2n-2. Alkynes are
traditionally known as acetylenes, although the name
acetylene also refers specifically to C2H2, known formally
as ethyne using IUPAC nomenclature.
1-butyne
2-chloro-1-butene
2,2 dichloro butane
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AROMATIC HYDROCARBONS
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SUBSTITUTION REACTIONS OF
AROMATIC HYDROCARBONS
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NOMENCLATURE OF ALKANES
The parent name of the molecule is determined by the number of carbons in the
longest chain.
In the case where two chains have the same number of carbons, the parent is the
chain with the most substituents.
The carbons in the chain are numbered starting from the end nearest the first
substituent.
In the case where there are substituents having the same number of carbons from
both ends, numbering starts from the end nearest the next substituent.
When more than one of a given substituent is present, a prefix is applied to indicate
the number of substituents. Use di- for two, tri- for three, tetra- for four, etc. and use
the number assigned to the carbon to indicate the position of each substituent.
3-METHYLHEPTANE
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Branched Alkanes
Branched substituents are numbered starting from the carbon of the substituent
attached to the parent chain. From this carbon, count the number of carbons in the
longest chain of the substituent. The substituent is named as an alkyl group based on
the number of carbons in this chain.
Numbering of the substituent chain starts from the carbon attached to the parent chain.
The entire name of the branched substituent is placed in parentheses, preceded by a
number indicating which parent-chain carbon it joins.
Substituents are listed in alphabetical order. To alphabetize, ignore numerical (di-, tri-,
tetra-) prefixes (e.g., ethyl would come before dimethyl), but don't ignore don't ignore
positional prefixes such as iso and tert (e.g., triethyl comes before tertbutyl).
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EXAMPLE 23.4
EXERCISE 23.4
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EXAMPLE 23.5
EXERCISE 23.5
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NOMENCLATURE OF ALKENES
Alkenes are named by dropping the -ane ending of the parent and adding -ene.
The parent structure is the longest chain containing both carbon atoms of the
double bond
Give the double bond the lowest possible numbers regardless of substituent
placement.
GIVE THE STRUCTURAL FORMULA AND NAME THE FOLLOWING
COMPOUND:
4-methyl-2-pentene
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EXERCISE 23.6
EXERCISE 23.7
EXERCISE 23.8
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NOMENCLATURE OF ALKYNES
Alkynes are named by dropping the -ane ending of the parent and adding -yne.
The parent structure is the longest chain containing both carbon atoms of the
double bond
Give the double bond the lowest possible numbers regardless of substituent
placement.
EXAMPLES:
EXERCISE 23.9
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NOMENCLATURE OF AROMATIC HYDROCARBONS
Benzene is the most common aromatic parent structure.
Multiple substituents on a benzene ring are numbered to give these
substituents the lowest possible numbers. When only two substituents are
attached to a benzene ring, they can be named by the common
nomenclature using ortho (o-) (1-2 placement), meta (m-) (1-3 placement)
or para (p-) (1-4 placement).
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EXAMPLES
1,2,4-trimethylbenzene
p-dimethylbenzene
2,3'-dimethylbiphenyl
EXERCISE 23.10
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Functional Groups of Organic compounds
ELIMINATION OF ALCOHOLS
Elimination
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NOMENCLATURE OF ALCOHOLS
PRIMARY ALCOHOL
SECONDARY ALCOHOL
TERTIARY ALCOHOL
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Skeletal formulae
 In a skeletal formula, all the hydrogen atoms are
removed from carbon chains, leaving just a carbon
skeleton with functional groups attached to it.
 For example; 2-butanol. The normal structural formula
and the skeletal formula look like this:
EXERCISE 23.11
NOMENCLATURE OF ETHERS
Ethers can be named by naming each of the two carbon groups as a separate
word followed by a space and the word ether.
The –OR group can also be named as a substituent using the group name,
alkoxy.
CH3–CH2–O–CH3
ethyl methyl ether
methoxyethane
cyclopentyl methyl ether
methoxycyclopentane
EXAMPLES:
2-pentyl 1-propyl ether
EXERCISE 23.12
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NOMENCLATURE OF ALDEHYDES
AND KETONES
Aldehydes and ketones are organic compounds which incorporate a
carbonyl functional group, C=O. The carbon atom of this group has two
remaining bonds that may be occupied by hydrogen or alkyl or aryl
substituents. If at least one of these substituents is hydrogen, the
compound is an aldehyde. If neither is hydrogen, the compound is a
ketone.
The IUPAC system of nomenclature assigns a characteristic suffix
to these classes, al to aldehydes and one to ketones
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EXAMPLES:
EXERCISE 23.13
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NOMENCLATURE OF CARBOXYLIC ACIDS
As with aldehydes, the carboxyl group must be located at the end of a carbon
chain. In the IUPAC system of nomenclature the carboxyl carbon is designated #1,
and other substituents are located and named accordingly. The characteristic
IUPAC suffix for a carboxyl group is "oic acid", and care must be taken not to
confuse this systematic nomenclature with the similar common system.
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Salicylic Acid (2-hydroxybenzoic acid)
EXERCISE 23.14
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NOMENCLATURE OF ESTERS
To name an ester you treat it as a derivative of an alcohol and an acid,
which it is.
First you name the part from the alcohol and then the part from the acid using
an -ate ending. The resulting name should be written as two words. This
diagram shows an ester that is made from methyl alcohol and acetic acid.
So its name is methyl acetate. Methyl from the methyl alcohol. Acetate from
the acetic acid.
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NOMENCLATURE OF ESTERS
Esters are named as derivatives of the carboxylic acid from which they are
formed. Condensation of ethanoic acid with methanol will produce methyl
ethanoate. As stated above the ending of the acid -oic is changed to -oate, much as if
the ester were a salt of the acid. The esterification reactions are generally easily
reversible by addition of water; the reverse reaction is called the hydrolysis of the ester
and proceeds in the presence of aqueous base.
Methyl ethanoate
CH3OH + CH3COOH
methanol + ethanoic acid
CH3COOCH3 + H2O
methyl ethanoate + water
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EXERCISE 23.15
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How Organic Reactions Occur:
Mechanisms
 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
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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
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Types of Steps in Reaction Mechanisms
 Bond formation or breakage can be symmetrical or
unsymetrical
 Symmetrical- homolytic
 Unsymmetrical- heterolytic
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Indicating Steps in Mechanisms
 Curved arrows indicate breaking and
forming of bonds
 Arrowheads with a “half” head (“fish-
hook”) indicate homolytic and
homogenic steps (called ‘radical
processes’)
 Arrowheads with a complete head
indicate heterolytic and heterogenic
steps (called ‘polar processes’)
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Radical Reactions
 Not as common as polar reactions
 Radicals react to complete electron octet of valence shell
 A radical can break a bond in another molecule and
abstract a partner with an electron, giving substitution in
the original molecule
 A radical can add to an alkene to give a new radical,
causing an addition reaction
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Steps in Radical Substitution
 Three types of steps
 Initiation – homolytic formation of two reactive
species with unpaired electrons
 Example – formation of Cl atoms form Cl2
and light
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Steps in Radical Substitution
 Propagation – reaction with molecule to generate
radical
 Example - reaction of chlorine atom with methane
to give HCl and CH3.
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Steps in Radical Substitution
 Termination – combination of two radicals to
form a stable product:
CH3. + CH3.  CH3CH3
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An Example of a Polar Reaction: Addition
of HBr to Ethylene
 HBr adds to the  part of C-C double bond
 The  bond is electron-rich, allowing it to function as a
nucleophile
 H-Br is electron deficient at the H since Br is much more
electronegative, making HBr an electrophile
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Mechanism of Addition of HBr to
Ethylene
 HBr electrophile is attacked by  electrons of ethylene
(nucleophile) to form a carbocation intermediate and
bromide ion
 Bromide adds to the positive center of the carbocation,
which is an electrophile, forming a C-Br  bond
 The result is that ethylene and HBr combine to form
bromoethane
 All polar reactions occur by combination of an electron-rich
site of a nucleophile and an electron-deficient site of an
electrophile
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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
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Rules for Using Curved
Arrows
 The arrow goes from the nucleophilic reaction site to the
electrophilic reaction site
 The nucleophilic site can be neutral or negatively charged
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Rules for Using Curved
Arrows
 The electrophilic site can be neutral or positively charged
 The octet rule must be followed
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Describing a Reaction: Energy Diagrams and
Transition States
 The highest energy point in
a reaction step is called the
transition state
 The energy needed to go
from reactant to transition
state is the activation
energy (DG‡)
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First Step in Addition
 In the addition of HBr
the (conceptual)
transition-state
structure for the first
step
 The  bond between
carbons begins to break
 The C–H bond begins
to form
 The H–Br bond
begins to break
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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
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A Comparison between Biological Reactions
and Laboratory Reactions
 Laboratory reactions usually carried out in organic
solvent
 Biological reactions in aqueous medium inside cells
 They are promoted by catalysts that lower the activation
barrier
 The catalysts are usually proteins, called enzymes
 Enzymes provide an alternative mechanism that is
compatible with the conditions of life
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