Electrophilic Addition

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  • Created by: LBCW0502
  • Created on: 01-01-18 18:01
What is an addition reaction?
Two reactants add together to form a single product (no side products produced)
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What is concentrated hydrochloric acid?
HCl (g) dissolved in water (until no more will dissolve)
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Why is hydrochloric acid a strong acid?
It is almost completely ionised and dissociated in water (no HCl present)
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How can a hydronium ion be described? (3)
An electron pair acceptor (Lewis acid), electrophile, proton donor (Bronsted acid)
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What is the nature of HCl?
It is an electrophile
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What is the nature of an alkene?
It is a nucleophile (electron pair donor/Lewis base)
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Describe the structure in an alkene
Contains a pi bond with high electron density, p-orbital, sp2 orbitals for carbon
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Are all Lewis bases nucleophiles?
Yes
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Are all strong bases nucleophiles?
Not always
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Are all Lewis acids electrophiles?
Yes
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Are all strong acids electrophiles?
Not always
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What is a reaction mechanism (3)?
A step by step account of what happens during a reaction. Shows all chemical species formed during the reaction. Shows bonds broken and formed with the use of curly arrows (link to resonance)
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What does a double-headed curly arrow show?
The movement of a pair of electrons
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What is an intermediate?
A chemical species formed in the reaction but is not a product
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What is the change in hybridisation of carbon atoms from an alkene to a halogenoalkane?
Change from sp2 hybridisation to sp3 hybridisation
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Proving the mechanism (experiment one)
No reaction between an alkene and aqueous sodium chloride. Chloride and sodium ions as well as water cannot start the reaction because they cannot react first
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Proving the mechanism (experiment two)
Reaction between an alkene and aqueous sodium chloride. If acid is added to one of the reagents, the reaction starts immediately. Hydronium ion reacts in the first step
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Proving the mechanism (experiment three)
Reaction between an alkene and aqueous sodium chloride. Adding more acid increases rate of reaction (lower pH values). Reaction doesn't work in basic solution. Rate of reaction depends on concentration of acid and concentration of alkene. H3O+ 1st
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Proving the mechanism (experiment four)
Reaction between alkene and HCl. Adding extra halide ions forms products. H and Cl not added in same single step. First step forms intermediate with halide ions
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What is a mechanistic family?
Various reactions can be classified into particular type of reactions
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Outline the general mechanism for electrophilic addition
Electrons transferred from pi bond in alkene to electrophile. Carbocation formed which nucleophile attacks. Product formed
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What is the slow step in electrophilic addition?
The formation of the carbocation
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What is the fast step in electrophilic addition?
Nucleophile attacking the carbocation to form the product
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Describe the energy profile for a one step reaction
Energy against progress of reaction (graph). One energy barrier (transition state/Ea). Starting materials at higher level, products at lower level (exothermic reaction/opposite for endothermic)
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What is the transition state on the energy diagram?
The high point in the energy profile, usually the midpoint of a reaction step
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What is a molecular species?
The chemical species that exists in the transition state with old bonds in bond breaking and new bonds in bond forming
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What are activated complexes?
They correspond to transition states and show the process of breaking and forming (bonds half formed or half broken)
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Describe the energy profile of a two step reaction
Level of starting materials > level for products. Two energy barriers/Ea (two transition states). The intermediate is between the transition states (has measurable lifetime). Activated complex/transient existence/no measurable lifetime
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What is the rate determining step?
The slowest step in the reaction sequence. A reaction cannot proceed faster than its slowest step (large energy barrier on energy diagram)
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What is regioselectivity?
One of the possible products is formed in larger amounts than the other one(s)
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What is regiospecificity?
Only one of the possible products is formed (100%)
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What is Markovnikov's rule?
Used to predict major product. When adding HX to a double bond, the H of HX goes to the C which already has the most Hs/least substituted C (empirical/observation)
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List the types of carbocations from the least favoured to the most favoured (lowest energy)
Methyl carbocation, primary carbocation, secondary carbocation, tertiary carbocation (most substituted carbocation is favoured). Due to inductive effect
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Which carbocation would form the fastest?
Tertiary carbocation (has low energy). Primary carbocation has high energy (slower formation). Tertiary carbocation would be the major product and primary carbocation would be the minor product
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If there is more than one carbocation intermediate possible, how will the reaction proceed?
Predominantly via the pathway that involves the lowest energy intermediate
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When Markovnikov's rule cannot be applied to a mechanism, how do you determine the product?
The reaction yields the product that results from the lowest energy carbocation (most substituted)
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When producing an alkyl hydrogen sulphate, what is the nucleophile?
Bisulphate ion (hydrogen sulfate)
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In a hydration reaction, what is regenerated?
Hydronium ion
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In the stereochemistry of additions, is there a preference when a water molecule is added?
No. H2O can be added either to the top or bottom face (no preference for either face)
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What is the test for unsaturation?
Bromine water turns from orange to colourless (reaction mechanism)
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Describe the polarisation of bromine
Uneven distribution of electrons in bromine molecule with one end positively charged and the other end negatively charged
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In stereospecificity, what is the product when a nucleophile is added in the syn position on a carbocation intermediate?
Leads to the formation of a cis compound
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In stereospecificity, what is the product when a nucleophile is added in the anti position on a carbocation intermediate?
Leads to the formation of a trans compound
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Is a C-C pi bond weaker than a C-C sigma bond?
Yes
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Why do simple alkenes not react with nucleophiles or bases?
Alkenes are electron rich (causes electron repulsion). Alkenes reacts with electrophiles
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How does syn addition take place?
When both X and Y are added from the same side (Cis)
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How does anti addition take place?
When X and Y are added from opposite sides (Trans)
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Why is HX polarised in electrophilic addition?
X is more electronegative than H, leading to a partial positive charge on H (attracted to electron rich double bond)
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Are addition reactions endothermic or exothermic?
Exothermic (two sigma bonds formed in product are stronger than sigma and pi bond broken in reactants)
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Is an alkene a Lewis base?
Yes
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Is HBr a Lewis acid?
Yes
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What is the rate determining step in the electrophilic addition of HX to an alkene?
The formation of a carbocation (fast step involves nucleophile attacking carbocation)
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Can trigonal planar atoms react with reagents from two directions with equal probability?
Yes
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Achiral starting materials yield which type of products?
Achiral or racemic products (an equal amount of two enantiomers)
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Hydrohalogenation occurs with which modes of addition of HX?
Syn and anti addition of HX
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Why is water added with sulphuric acid to an alkene to form an alcohol (hydration)?
Water is too weak an acid to protonate an alkene but can form H3O+ with sulphuric acid
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Does Markovnikov's rule apply to a hydration reaction?
Yes as well as syn/anti addition and carbocation rearrangements
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What is formed in the addition of halogen to an alkene?
A vicinal dilhalide
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What is the intermediate in halogenation?
Bridged halonium ion (not carbocation) which unstable/angle strain. Formed in the rate determining step
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Nucleophilic attack of X- occurs which side to form trans product?
Back side of molecule (anti addition of X2 across double bond)
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What is a stereospecific reaction?
Two specific stereoisomers of a starting material yields a particular stereoisomer of a product
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Which groups are attacked to an alkene in halohydrin formation?
X and OH
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Other cards in this set

Card 2

Front

What is concentrated hydrochloric acid?

Back

HCl (g) dissolved in water (until no more will dissolve)

Card 3

Front

Why is hydrochloric acid a strong acid?

Back

Preview of the front of card 3

Card 4

Front

How can a hydronium ion be described? (3)

Back

Preview of the front of card 4

Card 5

Front

What is the nature of HCl?

Back

Preview of the front of card 5
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