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Chemistry OCR A AS > Module 4.1.3 - Alkenes > Flashcards

Module 4.1.3 - Alkenes Flashcards

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1
Q

What are alkenes?

A

Unsaturated hydrocarbons (have at least one multiple bond between carbon atoms)

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2
Q

Why are alkenes unsaturated?

A

In alkanes, there is at least one double bond

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3
Q

What is the general formula for alkenes?

A

CnH2n

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4
Q

What bond is in a C=C bond?

A

Made up of 1 sigma bond and 1 pi bond (which restricts its rotation)

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5
Q

How is a sigma bond formed in an alkene?

A

Directly between two carbon atoms by the direct single overlap of orbitals directly between bonding atoms

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6
Q

How is a pi bond formed in an alkene?

A

The double, sideways overlap of p-orbitals ABOVE AND BELOW the bonding C atoms

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7
Q

What shape and bond angle is around the carbons in C=C?

A

Trigonal planar, 120 degrees
3 bonding regions of electrons which repel each other equally to get as far apart as possibly

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8
Q

How do you name alkenes?

A
  • Find the longest C chain that includes the alkene group & name the stem
  • Number parent chain from end to lowest number possible for alkene group
  • If more than 1 alkene group, use prefix (di-,tri-..) and add an ‘a’ at the end of stem of alkene
  • Name and number any alkene groups, with multiple list in alphabetical order by name
  • Cyclic alkenes have the prefix ‘-cyclo’
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9
Q

What is stereoisomerism?

A

Compounds with the same structural formula but with a different arrangement of the atoms in space.

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10
Q

What does E/Z stand for?

A

Entgegen/Zusammen

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11
Q

What is the criteria for E/Z stereoisomerism? (2)

A
  • Must have a carbon-carbon double bond, so the compound cannot rotate
  • EACH carbon atom of the C=C must have two different groups attached to it
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12
Q

How do you draw E/Z stereoisomerism?

A

Draw the molecule with its trigonal planar shape and attached groups are displayed in structural formula, has to condense only to the C=C

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13
Q

How do you find priority in alkene stereoisomers?

A

Compare the atoms that are attached on either side of the C=C and look at their atomic number (top and bottom)
Higher atomic number = Higher priority

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14
Q

How do you identify a Z-stereoisomer?

A

The highest priority groups are on the same side of the double bond (either both highest priority groups are top, top OR bottom, bottom)

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15
Q

How do you identify an E-stereoisomer?

A

Highest priority groups are on opposite sides of the C=C (one highest priority group on bottom, one on top)

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16
Q

What is the criteria for cis-trans stereoisomerism? (3)

A
  • must have a carbon-carbon double bond because it cannot rotate
  • Each carbon on the C=C must have two different groups attached
  • Two groups on the C=C must be identical
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17
Q

How do you identify a cis-stereoisomer?

A

The two same groups from each C atom are adjacent to the C=C

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18
Q

How do you identify a trans-stereoisomer?

A

Two same groups from each C atom are opposite the C=C bond (across from each other)

19
Q

Why are alkenes more reactive than alkanes?

A

Alkenes have C=C bond, which is their functional group

20
Q

What type of reactions do alkenes undergo?

A

Addition reactions - A group is added across the C=C double bond

21
Q

Why does the pi bond break when an alkene reacts?

A

It has relatively low bond enthalpy

22
Q

Why is the atom economy 100% for alkenes?

A

Since addition, only one product is formed so no waste product

23
Q

What are the different reactions for alkenes? (5)

A
  1. Bromination
  2. Chlorination
  3. Hydrogenation
  4. Hydration
  5. Hydrogen halide
24
Q

What is the test for identifying alkenes?

A

Bromine water (bromination) as it goes from orange to colourless

25
Q

What conditions and reagents are needed for hydrogenation?

A

Conditions: Nickel catalyst and 150 degrees C
reagents: alkene + H2
Forms an alkane

26
Q

What conditions and reagents are needed for bromination/chlorination?

A

Conditions: None
Reagents: Br2/Cl2 + alkene
Forms a haloalkane

27
Q

What conditions and reagents are needed for halogen halide?

A

Conditions: None
Reagents: alkene + HX (where X is a halogen)
Forms a haloalkane

28
Q

What conditions and reagents are needed for hydration?

A

Conditions: Conc. H3PO4 catalyst, high temperature & pressure, H2O has to be as a gas
Reagents: H2O (g) + alkene
Forms an alcohol

29
Q

What is electrophilic addition?

A

The mechanism that alkene reactions undergo, as C=C attracts electrophiles due to its high electron density

30
Q

What is an electrophile?

A

Electron pair acceptor

31
Q

How is the electrophilic addition mechanism shown?

A

Using curly arrow model (shows movement of electron pair)

32
Q

What must you show in the electrophilic addition mechanism?

A

The dipoles on the polar molecules and curly arrows showing where electrons are moving

33
Q

When is a mixture of two organic products formed from alkenes? (2)

A
  • When either hydrogen halide/steam reacts
  • Where alkene is unsymmetrical around the C=C double bond
34
Q

What are the two products formed from unsymmetrical alkenes catagorised as?

A

Major / Minor product

35
Q

What is Markownikoff’s rule?

A

The major product is the tertiary/secondary carbocation which is more stable than the primary carbocation (minor product), as the tertiary carbocation is the most stable and primary is the least stable

36
Q

What is the carbocation intermediate stage?

A

After the pi bond is broken, the carbocation is where one C atom has a positive charge and attracts the negative ion

37
Q

How many alkyls groups do each primary,secondary and tertiary carbocation have around it?

A

P - 1
S - 2
T - 3

38
Q

Which product is more likely to form?

A

Major product

39
Q

What are addition polymers?

A

Long, saturated molecules formed from alkene monomers

40
Q

What conditions and reagents are needed for addition polymerisation?

A

Conditions: high temperature and pressure + catalyst
Reagents: One type of alkene monomers

41
Q

What are the problems with disposing addition polymers? (2)

A
  • Non-biodegradable
  • Burning/Incineration of toxic chloro-gases
42
Q

In what ways are waste addition polymers processed which are more sustainable? (4)

A
  • Combustion for energy production
  • Removal of toxic waste products (like HCl)
  • Use as an organic feedstock
  • Recycles
43
Q

How can polymers be produced to minimise environmental damage? (2)

A
  • develop biodegradable polymers
  • develop photodegradable polymers

Chemistry OCR A AS (27 decks)

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