4.3 ALKENES Flashcards
Alkene double bond.
Alkene double bond:
-carbon atom has 4 electrons in outer shell for bonding
-for each C atom in a double bond, 3 electrons are used in sigma bonds
-last electrons is p-orbital perpendicular to plane atoms lie on
-p-orbitals from C atoms in a double bond overlap sideways = pi bond
-pi electron density lied above and below the plane of carbon atoms
-pi bond locks the carbon atoms in position, preventing rotation around the double bond
What is the difference between pi and sigma bond?
Difference between pi and sigma bond:
sigma- head on overlap of orbitals
pi- sideways overlap of p-orbitals
Reactivity of alkenes.
Reactivity of alkenes:
-alkenes more reactive than alkanes because of pi bond
-pi electrons more exposed because pi electron density lies above and below the plane of the bonding atoms
-pi bond with readily break as it is weaker than sigma bond
Addition reactions.
Addition reactions:
-characteristic reaction of alkenes
-occurs when pi bond breaks and two new atoms or groups are added to the molecule
What is halogenation?
Halogenation:
-bromination useful for distinguishing alkanes from alkenes
-orange colour of bromine turns colourless when Br reacts with double bond
What is hydrogenation?
Hydrogenation:
-addition of hydrogen to produce ethane
-ethane passes over a nickel catalyst at 423K
-all C=C bonds in an alkene would react if there is excess hydrogen
What is hydrohalogenation?
Hydrohalogenation:
-addition of gaseous hydrogen halides at room temperature to produce haloalkanes
What is hydration?
Hydration:
-addition of steam to produce ethanol
-an acid catalyst is used (phosphonic acid or concentrated sulfuric acid)
Electrophilic addition.
Electrophilic addition:
-alkene addition reactions from saturated compounds (mechanism is electrophilic addition)
-the double bond in an alkene represents a region of high electron density due to the presence of the pi-electrons. this region of high electron density attracts electrophiles
Electrophilic addition.
Electrophilic addition:
-alkene addition reactions from saturated compounds (mechanism is electrophilic addition)
-the double bond in an alkene represents a region of high electron density due to the presence of the pi-electrons. this region of high electron density attracts electrophiles
Electrophilic addition.
Electrophilic addition:
-alkene addition reactions from saturated compounds (mechanism is electrophilic addition)
-the double bond in an alkene represents a region of high electron density due to the presence of the pi-electrons. this region of high electron density attracts electrophiles
What are electrophiles?
Electrophiles are an electron pair acceptor that contain positive or partial negative change in structure
Addition of a polar molecule.
Addition of a polar molecule (e.g HCl with ethene):
1. Cl is more electronegative than H do HCl is polar and contains a dipole. HCl is an electrophile so is strongly attracted to the electrons in the pi bond
2. the electron pair in the pi bond is attracted to the partially positive H atom causing the double bond to break
3. a bond forms between H atom of H-Cl molecule and C atom that was a part of the double bond
4. the H-Cl bond breaks by heterolytic fission, with the electron pair going to the Cl atom
5. Cl ion and carbocation are formed
6. Cl ion reacts with the carbocation to form the addition product
What is Markowinoff’s rule?
Markiwinoff’s rule:
-the most stable carbocation will form the major product
-each alkyl group donates and pushes electrons towards the positive carboncation charge
-the more alkyl groups attached to the positive carbon centre, the more the positive charge is spread out so the more stable the ion is
Addition of a non-polar molecule.
Addition of a non-polar molecule:
1. Cl is a non-polar molecule, when it approaches an alkene the pi electrons cause polarisation of the Cl-Cl bond (induced dipole)
2. the electron pair in the pi bond moves towards the Cl delta positive end of the molecule causing the double bond to break and a bond to form between one of the carbon atoms from the double bond and a Cl atom
3. the Cl-Cl bond breaks by heterolytic fission with the electron pair going to the Cl delta positive end of the molecule
4. a Cl ion and a carbocation are formed
5. the Cl- ion reacts with the carbocation to form the addition product
