Alkenes

Question 1

what are alkenes?

Answer 1

hydrocarbons with the general formula (CnH2n)

Question 2

how do alkenes differ from the alkanes?

Answer 2

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)

Question 3

What does the bonding with alkenes contain?

Answer 3

-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

Question 4

what are hydrogens attached to carbon bonds by?

Answer 4

Sigma bonds

Question 5

what does the carbon to carbon sigma bond have?

Answer 5

-Electrons directly between the two carbon nuclei

Question 6

how was the pi bond formed?

Answer 6

By the sideways overlap of adjacent P orbitals above and below the bonding C atoms

Question 7

Where is where is the pi bond?

Answer 7

Above and below the plane of the molecule

Question 8

what is a carbon carbon double bond compromised of?

Answer 8

A sigma bond surrounded by a pi bond

Question 9

what is the geometry (shape) and bond angle around the carbon atoms?

Answer 9

-trigonal planar
-the bond angle is 120° (three bonding regions and zero lone pairs)

Question 10

what is structural isomerism?

Answer 10

molecules with the same molecular formula but different structural formula

Question 11

what are the four structural isomers of C4 H8?

Answer 11

-but-1-ene
-but-2-ene
-methylpropene
-cyclobutane

Question 12

what is what is stereoisomerism?

Answer 12

-stereoisomers are compounds with the same structural formula, but a different arrangement of atoms in space

Question 13

what is E/Z isomerism (geometric isomerism)?

Answer 13

-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

Question 14

why do we have E/Z isomerism?

Answer 14

-because of the restrict rotation around c=c

Question 15

when do we have E/Z isomerism?

Answer 15

must have c=c, each atom/group bonded or each of the carbons in c=c must be different

Question 16

how do you name alkenes?

Answer 16

-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

Question 17

what is an electrophile?

Answer 17

-an electron pair acceptor
-They are positive charge carriers (cation or polar)

Question 18

what are more reactive alkanes or alkenes?

Answer 18

alkenes

Question 19

What is the bond enthalpy of a pi bond compared to a sigma bond?

Answer 19

A pi bond has a lower bond enthalpy compared to a sigma bond

Question 20

where do alkenes contain a high electron density?

Answer 20

-within the double bond
-The electrons in the double bond attract electrophiles

Question 21

what are the reactions of Alkenes?

Answer 21

-addition of hydrogen, H2- hydrogenation
-Addition of halogen, X2- halogenation
-Addition of hydrogen halide, HX- halogenation
-Addition of water, H2O- hydration
-Addition of polymerisation

Question 22

what are the reagents and conditions of hydrogenation?

Answer 22

-Hydrogen (H2)
-Nickel (Ni) catalyst

Question 23

what is the importance of hydrogenation?

Answer 23

-the unsaturated alkene becomes saturated
-used in the manufacture of margarine (hydrogenating vegetable oils)

Question 24

What are the reagents and conditions of halogenation?

Answer 24

-Cl2, or Br2 or I2
-room temp

Question 25

what is the addition bromine (halogenation)

Answer 25

the Laboratory test for the presence of double bonds

Question 26

what is that addition of iodine used to do? (halogenation)

Answer 26

-Test for unsaturation in vegetable oils

Question 27

what is the chemical test for alkenes?

Answer 27

-add unsaturated compound into bromine -shake -bromine water turns colourless if double bonds are present

Question 28

What are the reagents and conditions for addition of hydrogen halides?

Answer 28

-HCl or Hbr or HI -Room Temp

Question 29

why will alkenes give rise to 2 products when reacting with hydrogen halides and other non-symmetrical molecules?

Answer 29

-Alkenes are unsymmetrical -For example propane reacts with hydrogen bromide to give to isomeric products

Question 30

How does the reactivity of the hydrogen halide increase?

Answer 30

-From HF to HI, following the order of the decreasing bond energy

Question 31

how does the reactivity of the haloalkanes increase?

Answer 31

Down the group as the strength of the hydrogen halide bond this weaker

Question 32

what are the reagents and conditions for hydration?

Answer 32

-steam (H2O (g)); (Temp> 100°C) -Phosphoric acid (H3PO4) catalyst

Question 33

What is the importance of hydration?

Answer 33

-addition of steam across a double bond is used in the industrial manufacture of alcohols

Question 34

What will the major product be in the mechanism for an unsymmetrical electrophile?

Answer 34

The major product will be the one which results from the most stable carbocation

Question 35

what is a carbocation?

Answer 35

Molecular ion in which a caron atom bears three bonds and a positive charge. There are three classifications of carbocations.

Question 36

What are the classifications of carbocations?

Answer 36

-Primary (carbocation bonded to 1 C atom) -Secondary (carbocation bonded to 2 C atoms) Tertiary (carbocation bonded to 3 C atoms)

Question 37

why is there an increase in stability in the three classifications of carbocations?

Answer 37

-due to the presence of an increasing number in alkyl groups

Question 38

what does the alkyl group do in terms of stability?

Answer 38

The alkyl group pushes the pair of electrons within the covalent bond towards the carbocation, making it ‘less’ positive, and more stable

Question 39

what is the major product dependent on in an electrophilic addition mechanism with an unsymmetrical, alkene and unsymmetrical small molecule?

Answer 39

The stability of the carbocation intermediates

Question 40

what is the major product formed from?

Answer 40

The most stable carbocation intermediate in the mechanism

Question 41

what is the minor product formed from?

Answer 41

The least stable carbocation intermediate in the mechanism

Question 42

what is the more stable a cation result in?

Answer 42

-The longer it exists in the reaction -This means there is a greater chance of it reacting with bromide ion

Question 43

What are polymers?

Answer 43

Long-chained molecules made by the joining of many small chained molecules called monomers

Question 44

what undergoes addition polymerisation?

Answer 44

alkenes

Question 45

why do alkenes undergo addition polymerisation?

Answer 45

To produce saturated chains, containing no double bonds

Question 46

What is broken and made in addition polymerisation?

Answer 46

A pie bond is broken, and two new sigma bonds are made

Question 47

What are addition polymers made from?

Answer 47

One type of monomer only

Question 48

What is the gen form of a cyclo alkene?

Answer 48

CnH2n-2