Alkenes

Question 1

Alkene bonds related to alkane

Answer 1

Alkene bonds are shorter and higher energy. React faster as less stable. Alkenes are unsaturated while alkanes are saturated

Question 2

Addition Reaction Outline

Answer 2

Conversion of alkene to alkane by adding more atoms (forming sigma bond) breaking pi bond in alkene and sigma bond in molecule being added. Reversible via elimination reaction

Question 3

Electrophile (electrophilic) Def

Answer 3

Electron poor substances. Attracted to negatively charged electrons

Question 4

Neutrophile (neutrophilic) Def

Answer 4

Electron rich substances. Attracted to positive charges eg cations

Question 5

Mechanism Def

Answer 5

Step-by-step account of what happens during a reaction. Shows all chemical species in reaction and all bonds broken and formed

Question 6

How to describe breaking/formation of bonds

Answer 6

Electron movement

Question 7

Rate Determining (Slowest) Step of an Addition Reaction

Answer 7

Addition of an electrophile (breaking of pi and sigma bonds)

Question 8

Carbocation

Answer 8

Intermediate of addition. Molecule with positively charged carbon atom (lost it’s electron). Triagonal planar geometry. 3 Types

Question 9

Primary Carboation

Answer 9

Positive carbon is attached to 1 other C. Least stable carbocation

Question 10

Secondary Carbocation

Answer 10

Positive C is attached to 2 other Cs. Intermediate stable carbocation

Question 11

Tertiary carbocation

Answer 11

Positive C is attached to 3 other Cs. Most stable carbocation

Question 12

What carbocation will always be formed in reaction (which pathway taken of more then 1 theoretically feesible)

Answer 12

Most stable

Question 13

Do different carbocations produce different products

Answer 13

yes, different arrangement of substituents

Question 14

Why do energy profile diagrams have multiple peaks

Answer 14

No. of transition states = no. of steps in reactions. 2nd transition state bonds go from 2D to 3D

Question 15

Markinov’s Rule

Answer 15

When a molecule containing hydrogen is added to an unsymmetrical alkene. H+ adds to C with greatest number of Hs already attached. Secondary atom (anion) attaches to other carbon

Question 16

What conditions lead to alkenes becoming alcohols

Answer 16

Water adding to alkene in the presence of an acid (H donating) catalyst and heat

Question 17

What happens to a catalyst if it binds to a molecule in 1 step

Answer 17

It is regenerated in another. Catalysts don’t get used up in reactions (usually seperating 1 H from H2O)

Question 18

Hydrogenation of Alkenes

Answer 18

Addition of H2 to alkene dissolved in suitable solvent with powdered, suspended metal catalyst. Metal catalysts break H2 bonds, alkene bonds to metal surface, H bonds to alkene. Atoms add on same side of double bond

Question 19

Heterogenous Catalysis Def

Answer 19

Catalysts not dissolved

Question 20

What does the conversion of an alkene to an alkane effect melting points

Answer 20

Melting point decreases

Question 21

What substances experience geometric isomerism

Answer 21

Substituted alkenes, substituted cycloalkanes and octahedral/square planar complexes with different ligands

Question 22

Geometric Isomer Def

Answer 22

Conversion between cis and trans isomers

Question 23

Why geometric isomerisation occurs in alkenes

Answer 23

No free rotation around sigma ( C - C bond) because of pi bond. Break pi bond, allow rotation around sigma bond and let pi bond reform. Energy needs to be provided

Question 24

Cis Isomer Def

Answer 24

2 identical groups on same side of double bond. Higher boiling point, lower melting point

Question 25

Trans Isomer

Answer 25

2 different groups on same side of double bond. Lower boiling point, higher melting point

Question 26

Under what conditions are they not considered geometric isomers

Answer 26

There must be 2 different substituents on all carbons

Question 27

Medical applications of alkene isomerisation

Answer 27

11-cis-retinal (pentene and aldehyde) combined with opsin (protein) in rod cells in cornea. When light hits retina, rotation around sigma bond at C11 and C12 cause conversion to trans state. Protein changes shape (ospin to rhodoposin). Nerve cells stimulated and brain registers image (enabling vision)

Question 28

Important Biological Alkenyl Compounds

Answer 28

Vitamin D3 (intestinal absorption of Ca, Fe, Mg and Zn) and Beta-Carotene (orange colour of carrots)

Question 29

Alkene synthesis from alkanes

Answer 29

Dehydrohalogenation. Elimination. Forming separate molecules (eg water) and alkene

Question 30

Dehydration of Alcohols

Answer 30

Alkene synthesis via elimination. Requires high conc of acidic conditions and heat. Tertiary alcohol is most reactive, primary is least