Substitution absorbs energy, Replace one H with a Br for ...
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2202 5 Organic Reactions 2017 Filled.notebook 1 June 05, 2017 Apr 256:18 PM Substitution A substitution reaction occurs when a hydrogen atom is removed from a hydrocarbon and replaced by a halide substituent. When a bromine molecule absorbs energy, the covalent bond is broken resulting in the formation of bromine atoms. These atoms are good examples of radicals very unstable and highly reactive particles. Ex 1: Replace one H with a Br for every mole of Br 2
Transcript of Substitution absorbs energy, Replace one H with a Br for ...
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Substitution A substitution reaction occurs when a hydrogen atom is removed from a hydrocarbon and replaced by a halide substituent.
When a bromine molecule absorbs energy, the covalent bond is broken resulting in the formation of bromine atoms. These atoms are good examples of radicals very unstable and highly reactive particles.
Ex 1:
Replace one H with a Br for every mole of Br2
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Substitution1
Substitution Reactions
HalogenRHRHalogenHalogen
+H Halogen
+
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2
Br
Ex 2:
Ex 3:
3Br
Br
Ex 4:
4Br
Br
Br
Bromine atoms can replace all four of the hydrogen atoms in methane but you will require 4 moles of BrBr (Br2).
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Ex 5: Structural isomerism is the existence of two or more structural formulas for one chemical formula.
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Ex 6: Draw and name the structural formulas for possible isomeric products of a reaction between propane and two molecules of chlorine. Answer Stage 1: The first chlorine molecule reacts with propane.
The products of the first step of the reaction are 1chloropropane or 2chloropropane.
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Stage 2a: If the second chlorine molecule reacts with 1chloropropane;
then the possible isomers are: 1,1dichloropropane, 1,2dichloropropane, and 1,3dichloropropane.
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Stage 2b: If the second chlorine molecule reacts with 2chloropropane;
then the possible isomers are: 1,2dichloropropane and 2,2dichloropropane.
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Benzene Benzene behaves chemically like an alkane (a saturated compound) and not like the alkenes and alkynes (unsaturated compounds.) The hybrid carbontocarbon bonds of the benzene ring are very stable and are not easily broken like the double and triple bonds
Since the hybrid bond is not easily broken, reactions between halogens and benzene result in the substitution of hydrogen with halogen atoms.
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Ex 7: Write an equation using structural formulas to illustrate a reaction between: a. benzene and chlorine
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b. one benzene molecule and two chlorine molecules
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Ex 8: Write an equation using structural formulas to illustrate a reaction between: a. methane and chlorine
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b. ethane and two moles of iodine
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Addition ReactionsAlkenes and alkynes are unsaturated hydrocarbons containing at least one double or triple bond. They do not undergo substitution reactions; instead, they undergo addition a reaction in which substituents are added to both carbons involved in the multiple bond.
Alkenes and alkynes are chemically more reactive than alkanes because of the presence of the multiple carbontocarbon bonds.
In alkenes, a double bond is reduced to a single bond and in alkynes, a triple bond is reduced to either a double or a single bond depending on the amount of the substituent available for addition.
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Addition
Addition Reactions
+
C C
YX
C C
X Y
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Ex 1: Predict the products of addition reactions involving: a. ethene and chlorine.
The product is 1,2dichloroethane.
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b. 1butene and chlorine.
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c. cyclohexene and fluorine.
The product is 1,2difluorocyclohexane.
The halogen atoms are added at the location of the double bonded carbon atoms. The reaction is spontaneous unlike substitution reactions, which require light energy to break the covalent bonds in the diatomic halogen molecules.
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Ex 2: Predict the products of addition reactions between: a. one molecule of ethyne and one molecule of chlorine
b. one molecule of ethyne and two molecules of chlorine
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Ex 3: Predict the product of a reaction between ethene and hydrochloric acid
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Ex 4: Predict the product of a reaction between ethene and bromine:
Ex 5: Predict the product of a reaction between propene and hydrogen:
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Ex 6: Predict the product of a reaction between propene and hydrobromic acid
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Ex 7: Predict the product of a reaction between ethene and hydrogen
Ex 8: Predict the product of a reaction between ethyne and two hydrogen
+ 2H2
Ex 9: Predict the product of a reaction between ethyne and hydrogen
+ H2
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An Application of the Addition Reaction
The decolourization of bromine water is an important test for the presence of a double carbontocarbon bond. The reaction is spontaneous. Bromine, which has a characteristic bright orange color does not react as easily with saturated hydrocarbons.
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Elimination Reactions : Halogen substituents can be removed from an alkyl halide in a reaction involving a base. The organic product of an elimination reaction is an unsaturated hydrocarbon.
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It removes a hydrogen from either carbon #1 or carbon #3. The hydroxide ion combines with the hydrogen to produce water, and the eliminated chlorine atom becomes a chloride ion.
Ex 1: Write a chemical equation to show the conversion of 2chlorobutane to an unsaturated hydrocarbon.
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+ KOH + H 2O + KBr
Ex 2: Write a chemical equation to show the conversion of 1bromoethane to an unsaturated hydrocarbon.
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+ NaOH + H 2O + NaCl
Ex 3: Write a chemical equation to show the conversion of 1chloropropane to an unsaturated hydrocarbon.
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Reactions producing Alcohols... Addition Reactions
The water molecule splits into hydrogen and a hydroxyl. These species are added to ethene at the location of the double bond. The product is ethanol.
Ex 1: Predict the product of a reaction between ethene and water
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Ex 2: Predict the product of a reaction between 2pentene and water
+ HOH
or
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Ex 3: Predict the product of a reaction between ethyne and two moles of water
1,2ethandiol, commonly known as ethylene glycol or radiator antifreeze
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Ex 4: Predict the product of a reaction between butene and water
+ HOH
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Reactions with Alcohols...Elimination Reactions: Alkenes can be produced by elimination of a water molecule from an alcohol. This reaction involves the use of an acid catalyst. A catalyst is a substance that speeds up a chemical reaction without being consumed. The acid catalyst in the example below is represented by the symbol H+.
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It should be obvious that a hydroxyl group and a hydrogen atom are being eliminated from the alkyl parent. The result is the formation of an unsaturated hydrocarbon and water.
Ex 1:Predict the product of a reaction between propanol and an acid catalyst.
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Ex2: Predict the product of a reaction between ethanol and an acid catalyst.
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Ex 3: Predict the product of a reaction between cyclohexanol and an acid catalyst
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Ex4: Predict the product of a reaction between 3pentanol and an acid catalyst
H+
An important application of this reaction type is the ripening of fruit in warehouses. The ripening process is stimulated when a plant produces ethene. If unripened fruit is stored under the right conditions, the production of ethene is inhibited allowing for long term storage. In order to initiate the ripening process, the fruit has to be exposed to ethene. This is where an elimination reaction involving ethanol becomes important.
+ H2O
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Esterification: Esters are produced when alcohols and carboxylic acids are reacted in the presence of an acid catalyst
Reactions producing Esters… Esterification
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Esterification
Esterification
H+
C O H
O
R(carboxylic acid)
+O HR'
(alcohol)
C O
O
R R'+
O HH
(ester)
(water)
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Ex 1: Predict the product for reaction between methanol and butanoic acid
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Ex 2: Predict the product for reaction between ethanoic acid and ethanol
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May 1710:24 PM
Ex 3: Predict the product for reaction between ethanoic acid and 1butanol
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Ex 4: Predict the product for reaction between of ethanoic acid and methanol.
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