Alkenes Flashcards
what are alkenes?
hydrocarbons with the general formula (CnH2n)
how do alkenes differ from the alkanes?
they have a to carbon=carbon double bond
This is a region of high electron density so they will react with electrophiles (electron electron pair acceptor)
What does the bonding with alkenes contain?
-A double bond which is made up of two distinct bonding regions the sigma bond and the pi bond
-The electrons are localised between two carbon atoms
what are hydrogens attached to carbon bonds by?
Sigma bonds
what does the carbon to carbon sigma bond have?
-Electrons directly between the two carbon nuclei
how was the pi bond formed?
By the sideways overlap of adjacent P orbitals above and below the bonding C atoms
Where is where is the pi bond?
Above and below the plane of the molecule
what is a carbon carbon double bond compromised of?
A sigma bond surrounded by a pi bond
what is the geometry (shape) and bond angle around the carbon atoms?
-trigonal planar
-the bond angle is 120° (three bonding regions and zero lone pairs)
what is structural isomerism?
molecules with the same molecular formula but different structural formula
what are the four structural isomers of C4 H8?
-but-1-ene
-but-2-ene
-methylpropene
-cyclobutane
what is what is stereoisomerism?
-stereoisomers are compounds with the same structural formula, but a different arrangement of atoms in space
what is E/Z isomerism (geometric isomerism)?
-arises from having two different groups on each carbon atom of the carbon-carbon double bond
-and the restricted rotation about the carbon-carbon double bond
why do we have E/Z isomerism?
-because of the restrict rotation around c=c
when do we have E/Z isomerism?
must have c=c, each atom/group bonded or each of the carbons in c=c must be different
how do you name alkenes?
-The ending for all alkenes is -ene
-for a carbon chain length longer than three carbon atoms, you must state the position of the double bond
what is an electrophile?
-an electron pair acceptor
-They are positive charge carriers (cation or polar)
what are more reactive alkanes or alkenes?
alkenes
What is the bond enthalpy of a pi bond compared to a sigma bond?
A pi bond has a lower bond enthalpy compared to a sigma bond
where do alkenes contain a high electron density?
-within the double bond
-The electrons in the double bond attract electrophiles
what are the reactions of Alkenes?
-addition of hydrogen, H2- hydrogenation
-Addition of halogen, X2- halogenation
-Addition of hydrogen halide, HX- halogenation
-Addition of water, H2O- hydration
-Addition of polymerisation
what are the reagents and conditions of hydrogenation?
-Hydrogen (H2)
-Nickel (Ni) catalyst
what is the importance of hydrogenation?
-the unsaturated alkene becomes saturated
-used in the manufacture of margarine (hydrogenating vegetable oils)
What are the reagents and conditions of halogenation?
-Cl2, or Br2 or I2
-room temp
what is the addition bromine (halogenation)
the Laboratory test for the presence of double bonds
what is that addition of iodine used to do? (halogenation)
-Test for unsaturation in vegetable oils
what is the chemical test for alkenes?
-add unsaturated compound into bromine
-shake
-bromine water turns colourless if double bonds are present
What are the reagents and conditions for addition of hydrogen halides?
-HCl or Hbr or HI
-Room Temp
why will alkenes give rise to 2 products when reacting with hydrogen halides and other non-symmetrical molecules?
-Alkenes are unsymmetrical
-For example propane reacts with hydrogen bromide to give to isomeric products
How does the reactivity of the hydrogen halide increase?
-From HF to HI, following the order of the decreasing bond energy
how does the reactivity of the haloalkanes increase?
Down the group as the strength of the hydrogen halide bond this weaker
what are the reagents and conditions for hydration?
-steam (H2O (g)); (Temp> 100°C)
-Phosphoric acid (H3PO4) catalyst
What is the importance of hydration?
-addition of steam across a double bond is used in the industrial manufacture of alcohols
What will the major product be in the mechanism for an unsymmetrical electrophile?
The major product will be the one which results from the most stable carbocation
what is a carbocation?
Molecular ion in which a caron atom bears three bonds and a positive charge. There are three classifications of carbocations.
What are the classifications of carbocations?
-Primary (carbocation bonded to 1 C atom)
-Secondary (carbocation bonded to 2 C atoms)
Tertiary (carbocation bonded to 3 C atoms)
why is there an increase in stability in the three classifications of carbocations?
-due to the presence of an increasing number in alkyl groups
what does the alkyl group do in terms of stability?
The alkyl group pushes the pair of electrons within the covalent bond towards the carbocation, making it ‘less’ positive, and more stable
what is the major product dependent on in an electrophilic addition mechanism with an unsymmetrical, alkene and unsymmetrical small molecule?
The stability of the carbocation intermediates
what is the major product formed from?
The most stable carbocation intermediate in the mechanism
what is the minor product formed from?
The least stable carbocation intermediate in the mechanism
what is the more stable a cation result in?
-The longer it exists in the reaction
-This means there is a greater chance of it reacting with bromide ion
What are polymers?
Long-chained molecules made by the joining of many small chained molecules called monomers
what undergoes addition polymerisation?
alkenes
why do alkenes undergo addition polymerisation?
To produce saturated chains, containing no double bonds
What is broken and made in addition polymerisation?
A pie bond is broken, and two new sigma bonds are made
What are addition polymers made from?
One type of monomer only
What is the gen form of a cyclo alkene?
CnH2n-2
