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A level- Chemistry > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

Describe how single bonds are formed

A

Sigma bonds are formed from end on overlap of atomic orbitals.
Thye are free to rotate.
Memorise diagram as well

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2
Q

Describe how double bonds are formed

A

Pi bonds- the 2nd bond forms is from side to side overlap of p-orbitals.
They are not free to rotate as it would break the overlap.
Memorise diagram

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3
Q

Where do the bonding electrons exist in the pi bond

A

Above and below the atom in 2 halves.

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4
Q

In a carbon bond is a sigma or pi bond stronger

A

Sigma bond

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5
Q

Define stereoisomers

A

They have the same structural formula but a different arrangement of atoms in space. They exist because C=C double bond cannot rotate.`

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6
Q

How can E/Z isomerism arise

A
  1. When there is restricted rotation- around a double bond- pi bond
  2. When each atom of the double bond has two different groups attached- non-symmetrical
  3. Non-rotation of the double bond leads to groups on either side of the carbon being fixed in poisition
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7
Q

Why do methyl-propene and but-1-ene not show E/Z isomerism

A

Because they have the same groups coming off them - symmetriacl so swapping then over gives the same molecule.

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8
Q

How do you decide whether a stereoisomer is E or Z

A
  1. For each carbon of C=C assign groups as high or low priority- CIP rules
  2. Higher atomic number=high priority
  3. Look at bonded atoms first
  4. If the same look at next atom in prority order
  5. If high priority is on the same side it is a Z isomer
  6. If on opposite sides it is an E isomer
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9
Q

Draw E/Z isomers of CH3CBr=CF(OH)

A

pack page 14 has answers

1-Fluoro-2bromoporp-1-en-1-ol is E

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10
Q

What is cis/trans isomerism

A

A special case of E/Z isomerism in which 1 of the two groups attained to each carbon of the double bond is the same.
If the same group on the same side of double bond-cis
otherwise-trans

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11
Q

Reaction of ethene with H2

A

Forms ethane

150 degrees and Ni catalyst

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12
Q

Reaction of ethene with Cl2 or any other halogen

A

Forms 1,2-dichloroethane

RT and pressure

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13
Q

Reaction of ethene with HBr or any other halogen

A

Forms 1-bromoethane

RT and pressure

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14
Q

Reaction of ethene with H2O

A

Forms ethanol

Steam and acid catalyst (H3PO4)

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15
Q

Describe what happens when you add bromine water and shake it with an equal volume of cyclohexane

A

On initial addition, the clear, colourless cyclohexane would form a separate layer on top of the orange bromine water.
On shaking, the two layers would remain, but the majority of the colour would be transferred into the upper (organic/cyclohexane) layer because bromine is more soluble in organic solvents than water.

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16
Q

Describe what happens when you add bromine water and shake it with an equal volume of cyclohexene

A

On initial addition, the clear, colourless cyclohexene would form a separate layer on top of the orange bromine water.
On shaking, the two layers would remain, but the orange colour would RAPIDLY disappear (decolourisation), leaving two clear, colourless layers.

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17
Q

How can bromine water be used to test for C=C

A

When added to an alkene the orange disappears - bromine undergoes an addition reaction and the double bond is broken
When added to an alkane there is no colour change

18
Q

Define electrophile

A

An atom which is attracted to an electron rich centre or atom where it accepts a pair of electrons to form a new covalent bond

19
Q

Defie addition reaction

A

A reaction in which a reactant molecule is added to an unsaturated molecule to make one saturated molecule.

20
Q

What does a curly arrow represent

A

It represents the movement of a pair of electrons

21
Q

Describes what happens in Electrophilic addition

A 
The HCl (or other reactant) molecule receives a pair of electrons from C=C- acting as an electrophile.
The HCl bond breaks with both electrons going to Cl - Heterolytic fission
22
Q

Draw mechanism between ethene and HBr

A

answer p25 of study pack
Ensure curly arrows start and finish in the correct places.
You have put the dipoles in- negative dipole goes to most electronegative atom.
You have shown the charges in the middle step.
And the pair of electrons.

23
Q

Define heterolytic fission

A

When a bond breaks and all the electrons go to the same atom forming a cation and anion.

24
Q

What is the intermediate in electrophilic addition called

A

A carbocation

25
Q

Describe Markownikoff’s rule

A
  1. The delta positive atom will tend to attach to C with fewer C-atoms attached- the edge usually
  2. Other end the delta negative will tend to attache to C with more C atoms attached.
    Major product is the product that follows this rule, minor is when it doesn’t
26
Q

Why does Markownikoff’s rule happen

A

Because the carbocation in the formation of the major product is more stable because it is attached to more C atoms.

27
Q

What is a primary, secondary and tertiary carbocation

A

Primary means the Carbon is attached to only 1 carbon- least stable
Secondary means it is attached to 2 carbons
Tertiary means it is attached to 3 carbons- most stable

28
Q

Write equation for percentage yield

A

Percentage yield= Actual yield/ theoretical yield *100

29
Q

Write equation for atom economy

A

Atom economy= Total (molar) mass of useful products/ total (molar) mass of all products *100

30
Q

Why is 100% yield rarely achieved in organic reactions

A
  1. Reaction may not go to completion
  2. It may be difficult to isolate the desired product from the reaction mixture
  3. Side products may be formed
31
Q

How can atom economy be improved

A
  1. Find a use for any other products

2. Change the synthesis reaction so that it produces fewer other products

32
Q

How can percentage yield be improved

A

Change the conditions so the reaction is more efficient

33
Q

What is an addition polymer

A

A very large molecular chain formed b the addition reactions of many unsaturated alkene molecules.

34
Q

Write equations to show the polymerisation of Hex-2-ene, and cyclohexene

A
  1. ensure there are dangly bonds sticking out

2. Brackets around the product with n on the outside

35
Q

Why are polymers difficult to get rid of

A

They are non-biodegradeable

36
Q

Give advantage and disadvantage of recycling polymers

A

Adv- reduces environmental impact by conserving finite fossil fuels and decreasing amount of waste going to landfill.
Disadv- They have to be sorted-very time consuming

37
Q

Poblems with the recycling of PVC

A

It is hazardous due to the high chlorine content present in the polymer.
Dumping PVC in landfill is not sustainable.
When burnt PVC releases hydrogen chloride a corrosive gas and other pollutants.
So it is dissolved using solvents and is recovered by precipitation of the solvent.

38
Q

Give advantage and disadvantage of using waste polymers as fuel

A

Adv- can be used to generate electricity when incinerated.

39
Q

Give advantage and disadvantage of feedstock recycling

A

Adv- can handle unsorted and unwashed polymers

produces materials that can be used as raw materials form the productions of new polymers

40
Q

What are biodegradable polymers and their advantages

A

Are broken down by microorganisms into water, CO2 and biological compounds.
Are made from starch or cellulose or contain additives.
Bioplastics are likely to be more extensively used in packaging, electronics and more fuel-efficient and recyclable vehicles.

41
Q

What are photodegradable polymers and their advantages

A

They are oil-based polymers which contain bonds that are weakened by absorbing light to start the degradation.

A level- Chemistry (31 decks)

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