3.3.4 Alkenes Knowledge Flashcards
what are alkenes?
hydrocarbons with one or more C=C double bonds
what is the shape around C atoms in a C=C bond and why?
C atoms in C=C double bonds have 3 bonds in total, so the shape around the double bond is planar and the bond angle is 120
what are the 2 bonds that make up a C=C bond?
sigma and pi
what is a single covalent bond made from, in terms of orbitals?
The single bond is made from a sigma overlap; an s orbital from each carbon overlap and their electrons share the region of space between the orbitals
what is a double covalent bond made from, in terms of orbitals?
The double bond is made from sigma and pi overlap; a pi overlap is when p orbitals overlap above and below the carbons
are single or double covalent bonds stronger?
double
are sigma or pi bonds stronger?
sigma
are sigma or pi bonds stronger, and which is more reactive?
A sigma bond is stronger than a pi bond, which means pi bonds are more reactive (easier to break)
why are alkenes more reactive than alkanes?
- Double bonds are stronger than single
- A sigma bond is stronger than a pi bond, which means pi bonds are more reactive (easier to break)
- alkanes have only single bonds
what reagent is used in the chemical test for alkenes?
bromine
what are the observations for the chemical test for alkenes?
Bromine is an orange liquid; the haloalkanes formed when bromine reacts with an alkene are colourless.
write a balanced equation for the reaction that occurs in a chemical test for alkenes, of ethene
H2C=CH2 + Br2 ⇒ CH2BrCH2Br
what is the reagent required for the electrophilic addition of hydrogen halides and alkenes?
HCl or HBr
what are the conditions required for the electrophilic addition of hydrogen halides and alkenes?
none
what is the product of the electrophilic addition of hydrogen halides and alkenes?
haloalkane
what is the electrophile in the electrophilic addition of hydrogen halides and alkenes?
HBr or HCl, depending on which is sued
what is the reagent required for the electrophilic addition of halogens and alkenes?
Cl2 or Br2
what are the conditions required for the electrophilic addition of halogens and alkenes?
none
what is the product of the electrophilic addition of halogens and alkenes?
haloalkane
why does bromine react with an alkene, even though bromine is a non-polar molecule?
The C=C bond is electron rich and induces a dipole in Br2; the delta positive Br is attracted to the C=C double bond.
what is the reagent required for the electrophilic addition to create an alkyl hydrogensulfate?
H2SO4
what are the conditions required for the electrophilic addition of alkenes and H2SO4?
none
what is the product of the electrophilic addition of alkenes and H2SO4?
alkyl hydrogensulfate
draw the displayed formula for H2SO4
draw the mechanism for the reaction between H2SO4 and propene
electrophilic addition
draw the mechanism for the reaction between bromine and ethene
electrophilic addition
draw the mechanism for the reaction between HCl and ethene
electrophilic addition
what is a carbocation?
a positive molecular ion (cation) where the charged atom is carbon
what is the positive inductive effect?
the ability to lose electron density through a covalent bond
what is the reactive intermediate in an electrophilic addition reaction of hydrogen halides?
carbocation
what are the qualities of reactive intermediates?
Reactive intermediates are unstable and quickly either react further or decompose into their reactants
what does the stability of a carbocation depend on?
Carbocations have different stabilities depending on the number of alkyl groups bonded to the positive C atom
why does the number of alkyl groups bonded to the C+ atom affect a carbocation’s stability?
Alkyl groups push electron density onto whatever they are bonded to; positive inductive effect
why are tertiary carbocations more stable than primary?
because there is a greater positive inductive effect from 3 alkyl groups in tertiary carbocation, than 1 alkyl group in primary carbocation
will the major or minor product of electrophilic addition form in greater quantity?
major
in the electrophilic addition of hydrogen bromide and prop-1-ene, what will be the major and minor product and why?
2-bromopropane is formed from a secondary carbocation, where 1-bromopropane is formed from a primary carbocation.
The secondary carbocation is more stable than the primary due to the positive inductive effect. Therefore, 2-bromopropane is the major product.
what is the mechanism to make an alcohol from an alkyl hydrogen sulfate?
nulcophilic substitution/hydrolysis
what are the 2 steps to make an alcohol from an alkyl hydrogen sulfate?
- Step 1: alkene + H2SO4 ⇒ alkyl hydrogensulfate
- Step 2: alkyl hydrogensulfate + H2O ⇒ alcohol + H2SO4
what is the role of the sulfuric acid in the nucleophilic substitution of an alkyl hydrogen sulfate to make an alcohol?
catalyst
out line a mechanism to make ethanol from ethene using H2SO4
what is the reagent required for the direct hydration of an alkene?
h2o(g) - steam
what conditions are required for the direct hydration of an alkene?
concentrated H3PO4 catalyst
what is the product of the direct hydration of an alkene?
alcohol
outline a mechanism for the direct hydration of ethene
