T8: Alkenes Flashcards
alkenes are molecules with…
1 or more C=C double bonds
alkenes general formula is C atoms in C=C double bonds have 3 bonds in total which means that the shape around C=C double bonds is what and the bond angle is what
shape = planar
bond angle = 120°
C-C single bond: composed of what
σ bond
C-C single bond: bond enthalpy / kJ mol-1
+347
C=C double bond: composed of what
σ and π bond
C=C double bond: bond enthalpy / kJ mol-1
+612
what is a σ bond and a π bond
σ bond the same as a regular single covalent bond and it is added to π bond that turns a single bond into a double bond
which is stronger, a C-C or C=C bond?
C=C
which is stronger, a sigma or pi bond?
σ bond stronger
(612 - 347 = 265)
which is more reactive, a sigma or pi bond?
π more reactive
(needs less energy to break)
which is more reactive an alkane or an alkene?
alkenes are more reactive
(thermal cracking is used to turn alkanes into alkenes)
orbital definition
a region of space where up to 2 electrons can be formed
covalent bonds are made from
shared pairs of electrons
a C=C double bond is made from
2 shared pairs of electrons
how is the π bond formed
P orbital overlaps sideways on to form a π bond
why is there no rotation around a C=C double bond
the π bond is formed by overlap of p orbitals
if there was any rotation, the p orbitals would no longer overlap and the π bond would break
C=C are areas with…
high electron density (e- rich -> 4e- between 2 C atoms)
electrophile definition
an electron pair acceptor
3 types of electrophiles
- hydrogen halides (HCl, HBr)
- halogens (Cl2, Br2)
- sulphuric acid (H2SO4)
electrophilic addition with hydrogen halides: reagent
HCl, HBr
electrophilic addition with hydrogen halides: conditions
none
electrophilic addition with hydrogen halides: type of product
haloalkane
electrophilic addition with hydrogen halides: the electrophile in this reaction is…
HBr
electrophilic addition with hydrogen halides: this reaction is an addition reaction because…
a H atom and a Br atom have been added
electrophilic addition with hydrogen halides: overall reaction
CH2=CH2 + HBr -> CH3CH2Br
what is formed in the middle step of electrophilic addition with hydrogen halides
an intermediate carbocation
what are alkyl groups
Methyl (CH3), Ethyl (CH2CH3), Propyl (CH2CH2CH3)
what is a positive inductive effect
the ability to release electron density through a covalent bond
stability: carbocation rank
primary carbocation (least stable)
secondary carbocation
tertiary carbocation (most stable)
why are tertiary carbocations more stable than primary carbocations
there is a positive inductive effect from 3 alkyl groups in 3° carbocation compared to 2 alkyl groups in 2° carbocations
explain why 2-bromopropane is the major product in this reaction (outline the mechanism to form both possible products when propane reacts with HBr)
2-bromopropane is made from a secondary carbocation
1-bromopropane is made from a primary carbocation
secondary carbocations are more stable than primary carbocations due to the positive inductive effect of the 2 alkyl groups compared to 1
electrophilic addition with halogens: reagent
Cl2, Br2
electrophilic addition with halogens: conditions
none
electrophilic addition with halogens: type of product
haloalkane
electrophilic addition with halogens: bromine reacts with alkenes even though bromine is a non-polar molecule. explain why bromine reacts with alkenes
the C=C bond is e- rich and induces a dipole in the Br2
partially positive Br attracted to C=C double bond
electrophilic addition with halogens: overall reaction
CH2=CH2 + Br2 -> CH2BrCH2Br
reagent used to test for alkenes
Br2
observation when alkene present
orange to colourless
observation when alkene not present
no visible change
structural formula for H2SO4
H
|
OSO3H
electrophilic addition with sulphuric acid: reagent
H2SO4
electrophilic addition with sulphuric acid: conditions
none
electrophilic addition with sulphuric acid: type of product
alkyl hydrogen sulphate
electrophilic addition with sulphuric acid: overall reaction
CH2=CH2 + H2SO4 -> CH3CH2OSO3H
this is 1 way to make alcohols from alkenes in a 2-step process: step 1
alkene + H2SO4 -> alkyl hydrogen sulphate
this is 1 way to make alcohols from alkenes in a 2-step process: step 2
alkyl hydrogen sulphate + H2O -> alcohol + H2SO4 (hydrolysis)
role of the H2SO4 in the 2-step transition from alkene to alcohol
H2SO4 is reproduced so it is called a catalyst
direct hydration: reagent
H2O(g)
direct hydration: conditions
conc H3PO4 catalyst
direct hydration: type of product
alcohol
