Organic Chemistry: Alkenes

Question 1

what is a sigma bond?

Answer 1

overlap of orbitals directly between the bonding atoms

Question 2

what is a pi bond?

Answer 2

formed by the sideways overlap of adjacent p orbitals above and below the bonding C atoms

Question 3

what are stereoisomers?

Answer 3

compounds with the same structural formula but with different arrangements of the atoms in 3 dimensional space

Question 4

what are cahn-ingold-prelog rules?

Answer 4

• the prefix e- or z- is based of the atomic numbers of the atoms
• e-: the atoms with the highest atomic number are on opposite sides
• z-: the atoms with the highest atomic number are on the same side of the double bond

Question 5

What is e/z isomerism?

Answer 5

e-: the atom with highest atomic number are on opposite sides
z-: the atom with the highest atomic number are on the same side

Question 6

What is cis/trans isomerism?

Answer 6

• cis: the same group attached is on the same side of the bond
• trans: the same group attached is on opposite sides of

Question 7

what are the 3 types of reaction for alkenes?

Answer 7

• combustion
• electrophilic addition
• addition polymerisation

Question 8

why is the combustion of alkenes unusual?

Answer 8

lots of other uses due to their reactivity

Question 9

what is the reaction to go from an unsaturated alkene to saturated alkane?

Answer 9

• addition of hydrogen
• NIckel catalyst
• 150 degrees celsuis

Question 10

what is addition of halogens to alkenes called?

Answer 10

halogenation - makes a di-substitued haloalkane

Question 11

how do you test for alkenes?

Answer 11

bromine water added to alkene - Br2(aq) - orange to colourless

Question 12

what does the addition of hydrogen halides to alkenes form?

Answer 12

haloalkane

Question 13

why is the addition of hydrogen halide to alkenes a possible problem?

Answer 13

• alkenes are unsymmetrical
• so a mixture of products are usually formed

Question 14

what is the reaction of an alkene with steam?

Answer 14

alkene + steam –> alcohol

Question 15

what is the formula for steam?

Answer 15

H2O(g)

Question 16

what acid catalyst is used when alkenes react with steam?

Answer 16

H3PO4 - phosphoric acid

Question 17

what is a polymer?

Answer 17

long molecular chain built up from monomers

Question 18

what is a monomer?

Answer 18

small molecule that combines with many other monomers to form a polymer

Question 19

what is addition polymerisation?

Answer 19

formation of a very long molecular chain by repeated addition reaction of many unsaturated alkene molecules

Question 20

what are biodegradable polymers?

Answer 20

• made from natural materials
• reduce reliance on crude oil
• naturally decompose

Question 21

what are photodegradable polymers?

Answer 21

• designed to contain bonds that are weakened by absorbing light - produces reactive radicals

Question 22

what type of rotation do pi bonds have?

Answer 22

restricted rotation

Question 23

what shape are alkenes?

Answer 23

• trigonal planar, 120 around each carbon in C=C of alkenes

Question 24

what are alkenes?

Answer 24

• unsaturated hydrocarbons containing a C=C bond comprising a pi bond and a sigma bond

Question 25

what impacts the reactivity of alkenes?

Answer 25

relatively low bond enthalpy of the pi bonds

Question 26

what are the differences between pi bonds and sigma bonds?

Answer 26

• pi bonds are above and below bonding atoms whereas sigma bonds are between the bonding atoms
• pi bonds have sideways overlap whereas sigma bonds have direct overlap
• pi bond has lower bond enthalpy than sigma bond

Question 27

what are advantages of using polymers?

Answer 27

• combustion for energy production
• use as an organic feedstock
• removal of toxic waste products

Question 28

why may a compound not have E/Z isomers?

Answer 28

one carbon atom in the double bond is attached to two groups which are identical