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Chemistry > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

Describe the structure and bonding in alkenes

A

Structure: trigonal planar , 120°

Bonding : double carbon bond (pi and sigma)

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2
Q

Explain what a pi bond is

A

Side to side overlap of p orbitals in plane above and below sigma bond

Electron rich centre

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3
Q

State the general formula of alkenes

A

CnH2n

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4
Q

Why might the formula of an alkene not fit the general formula?

A

Diene or cyclic

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5
Q

Define stereoisomerism

A

Same structural formula but different orientation in space

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6
Q

Why does stereoisomerism not arise in alkanes?

A

Don’t have pi bond which restricts rotation

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7
Q

What must a compound have to display stereoisomerism?

A

Double carbon bond

Two different groups attached to each carbon atom involved in double bond

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8
Q

Give the types of E/Z isomerism

A

E= trans (H on opposite sides of c=c)

Z= cis (H on same side of c=c)

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9
Q

How do you determine if a compound has E/Z(cis/trans) isomerism?

A

Has c=c
Has two different groups on each carbon atom

Highest priority group on opposite sides = E

On Same side= Z

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10
Q

How do you determine if a compound has cis/trans isomerism?

A

Has one H bonded to each carbon atom

Cis= H on same side
Z=H on opposite sides

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11
Q

How is priority assigned to groups?

A

Atom with highest atomic number (proton)

If the same do next atom along with highest atomic number

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12
Q

What is the name for the method of assigning priority?

A

Cahn-ingold-prolog principle

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13
Q

Do E or Z isomers have a higher BP, why?

A

E

Fit more closely and compact so have more surface contact points so London forces are stronger

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14
Q

Describe and explain the reactivity of alkenes

A

Fairly reactive

In pi bond e are above and below the plane of the sigma bond so more exposed

Pi have a low bond enthalpy so are easily broken

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15
Q

State the addition reactions possible with alkenes

A

Hydrogenation

Elecrrophillic addition:
Halogenation
Addition of hydrogen halide

Hydration

Polymerisation

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16
Q

State conditions, products and reagents for hydrogenation

A

Nickel catalyst

Hydrogen

Alkane

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17
Q

State conditions, products and reagents for hydration

A

Acid catalyst

Steam (water at high temp)

Mixture of alcohols

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18
Q

Describe the test for alkenes

A

Add bromine water

Positive= orange to colourless

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19
Q

What is Halogenation?

A

Electrophillic addition

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20
Q

What is an electrophile?

A

Species that accepts a pair of electrons

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21
Q

Describe electrophillic addition of Br2

A

Br2 approaches double bond

Electron rich Centre induces dipole

Pi bond breaks donating 2xe to positively partially charged Br

Br2 breaks via heterolytic fission producing two ions

Br+ bonds to carbon producing intermediate (carbonation)

Br- bonds to carbocation by donating lone pair of electrons to form drive covalent bond

haloakane is formed

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22
Q

Define carbocation

A

Positively charged carbon

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23
Q

What do curly arrows show?

A

Movement of e from donator to reciever

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24
Q

At what temperature does electrophillic addition occur at?

A

Rapid at room temperature

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25
Q

How is a liquid alkene and gaseous hydrogenhalide reacted?

A

Gas bubbled through

26
Q

How is a liquid alkene and liquid hydrogenhalide reacted?

A

Mixed

27
Q

How is a gaseous alkene and gaseous hydrogenhalide reacted?

A

Mixed

28
Q

Name the two types of product that form from the reaction between an unsymmetrical alkene and hydrogenhalide

A

Major and minor

29
Q

How are major and minor products classified?

A

Major = most stable (highest carbocation)

30
Q

How are carbocations classified?

A

The number of carbon atoms bonded to carbocation

1° 2° 3°

31
Q

What carbocation is the most stable and why? (name effect)

A

3° because has the most alkyl groups which push electrons towards c+ stabilising it more

+ is spread over alkyl groups so more alkyl groups = more stable

Inductive effect

32
Q

What is markownikoffs rule?

A

Hydrogen in hydrogen halide will bond to the carbon with the most hydrogens in an unstable alkene to produce the most stable carbocation and major product

33
Q

Outline a Polymerisation reaction

A

Monomers are joined to form a polymer with identical repeating units

34
Q

State the conditions in polymerisation

A

High temp and pa

Catalyst

35
Q

What are the environmental concerns of polymers?

A

Not very reactive so don’t degrable easily

Persist in environment= destroy marine life

36
Q

State the different methods of processing polymers

A

Recycling
Chemical feedstock
Energy production

37
Q

Describe how polymers are recycled

A

Polymers have to be sorted

Chopped into flakes

Washed, dried and melted to produce pellets

Used to produce more polymer products

38
Q

Describe how polymers are used for energy production

A

Polymers are burnt and release lots of energy

Energy boils water to steam which turns turbines to generate electricity

39
Q

Why might polymers be use dto produce electricity ?

A

Can’t be recycled

40
Q

Describe how polymers are Used as chemical feedstock

A

Polymers don’t have to be sorted

Chemical and thermal processes are used to produce oil gas or monomers

Use to produce new polymers

41
Q

What is the advantage of using polymers for chemical feedstock and not recycling?

A

They don’t have to be sorted

42
Q

What’s the difference between chemical feedstock and recycling?

A

In Chemical feedstock the polymers don’t have to be washed

Raw materials are produces in chemical feed stock but polymer pellets in recycling

43
Q

What is a bioplastic?

A

A plastic made from starch, cellulose, plant oil or proteins

44
Q

What are the advantages of using bioplastic?

A

Don’t produce toxic byproducts when degraded
Conserve oil reserves
Sustainable and renewable

45
Q

Define biodegradable

A

Degraded by micro-organisms to produce co2 water and biological compounds

46
Q

What are biodegrable plastics usually made of?

A

Cellulose or starch

47
Q

What are the advantages of using biodegrable plastics?

A

Produce no toxic byproducts

48
Q

What are photodegradable plastics?

A

Plastics where the bonds are weakened by light to start the degradation process

49
Q

Why are photodegradable plastics not as environmentally friendly as biodegrable plastics?

A

May be made of oil (not sustainable, renewable and may produce toxic byproducts)

50
Q

What is a polymer?

A

A large molecule made of many identical repeating units called monomers

51
Q

What is a Co-polymer

A

A polymers made of at least 2 different monomers

52
Q

Describe the difference between the strength of long and short polymers

A

Longer = stronger

53
Q

Describe and explain how branching affects strength of polymers

A

Less branching = stronger as more compact

54
Q

What is cross-linking?

A

Covalent bonds between chains

55
Q

Define thermosetting polymers

A

Polymers with cross links

56
Q

Describe thermosetting polymers

A

Retain shape when heated

57
Q

Define thermoplastic polymers

A

Have no crosslinks, only intermolecular forces

58
Q

Describe thermoplastics

A

Change shape when heated (remoulded)

59
Q

Describe and explain the effect of crosslinks on polymers

A

More crosslinks = strogner and so don’t change shape as much when heated

60
Q

Why does cyclo hexane have a BP than cyclopropane

A

More carbon atoms means more surface contact points means stronger LF which require more energy to break

Chemistry (28 decks)

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