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chemistry > organic chemistry - alkenes > Flashcards

organic chemistry - alkenes Flashcards

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1
Q

What is the general formula for alkenes?

A

CₙH₂ₙ

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2
Q

What is meant by ‘unsaturated’?

A

A molecule that contains a carbon-carbon double bond (C=C)

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3
Q

What types of bond make up a double bond? And describe each of them.

A

It is made of a σ-bond and a π-bond.

The sigma bond is between the two carbons.

Adjacent p-orbitals above and below the bonding C atoms overlap, creating a pi bond.

(pi bond surrounds sigma bond)

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4
Q

What is a sigma bond?

A

The result of the overlap of two orbitals, one from each atom. It has two electrons that are shared between the bonding atoms. They are single covalent bonds.

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5
Q

What is the bond angle around the double bond?

A

3 bonding regions, so 120° with trigonal planar shape

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6
Q

What is positional (or structural) isomerism?

A

Different isomers formed when the C=C appears between different carbon atoms.

(number added before -ene to indicate first carbon involved)

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7
Q

What are stereoisomers?

A

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

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8
Q

Why do you get stereoisomers?

A

Because the double bond locks the carbon atoms in place and prevents free rotation.

(their geometry is different to alkanes, where rotation is possible around every atom)

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9
Q

What is E/Z isomerism?

A

A type of stereoisomerism:

  • E is for when the same group is on opposite sides
  • Z is when the same groups are on the same side.
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10
Q

What are conditions for stereoisomerism?

A

molecules must:

  • have a C=C double bond
  • have different groups attached to each carbon in the double bond.
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11
Q

In E/Z isomerism, what happens when there are more than 2 different groups?

A

Each atom (directly) attached to the carbon in the C=C is given a priority based on its atomic number. If first atom is the same, look for the first point of difference.

(Higher priority adjacent -> Z isomer, diagonal -> E isomer)

(Cahn-Ingold Prelog Rules)

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12
Q

What is cis/trans isomerism?

A

A subset of E/Z isomerism. At least two of the groups attached to the carbon need to be the same. (usually hydrogen)

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13
Q

What is an addition reaction?

A

A reaction where 2 molecules react together to produce one

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14
Q

What is hydrogenation of alkenes?

A

Alkenes react with hydrogen to produce their equivalent alkane

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15
Q

What catalyst is needed for hydrogenation of of alkenes?

A

nickel catalyst (and temp. 150°C)

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16
Q

What is hydration of alkenes?

A

Alkenes are converted to alcohols by reacting with steam

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17
Q

What catalyst is needed for hydration of alkenes?

A

An acid catalyst eg. phosphoric acid

reaction is reversible

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18
Q

What is halogenation of alkenes?

A

Alkenes react with halogen molecules to form dihaloalkanes (no catalyst needed)

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19
Q

What is the test for alkenes?

A

Bromine water goes from orange to colourless

20
Q

What is the reaction of alkenes with hydrogen halides?

A

Forms haloalkanes (room temp, gaseous)

21
Q

What is electrophilic addition?

A

The mechanism alkenes use for addition reactions

22
Q

What is an electrophile?

A

An atom or group of atoms that accepts an electron pair

and is attracted to an electron-rich centre) (usually positive ion or atom with δ+ charge

23
Q

Describe step 1 of electrophilic addition with hydrogen halides.

A

The hydrogen halide has a permanent dipole (because the halogen is more electronegative than H): H is δ+ and halogen is δ-

Draw curly arrow from hydrogen halide bond to halogen - shows electrons moving to δ- atom.

Draw another curly arrow from double bond to δ+ Hydrogen. Double bond breaks because electrons are more attracted to δ+ atom and a bond is formed between C and H.

24
Q

Describe step 2 of electrophilic addition with hydrogen halides

A

Indicate positive charge on the other carbon atom that was in the double bond. It is now a carbocation.

Draw halide ion with lone pair, and draw curly arrow from lone pair to carbocation. They react to form the addition product.

25
Q

How does electrophilic addition work with halogen molecules?

A

The molecule needs to be polarised as halogen molecules aren’t polar. An induced dipole is formed.

The electrons in the pi bond repel the electrons in the halogen molecule - so the closer atom becomes δ+ and the further one becomes δ-

Then electrophilic addition can occur.

26
Q

Multiple products can be made when hydrogen halides react with unsymmetrical alkenes. How do we know which is the major product/minor product?

A

The hydrogen attaches to the carbon with larger no. of hydrogens and smaller no. of carbons.

(Markownikoff’s Rule)

27
Q

Describe different carbocations

A

primary carbocation (1°) has 1 carbon chain attached

secondary carbocation (2°) has 2 carbon chains attached

tertiary carbocation (3°) has 3 carbon chains attached

primary is least stable, then secondary, and tertiary is most stable.

28
Q

What is an alkyl group?

A

carbon and hydrogen chain

29
Q

Why do carbocations get more stable as they are attached to more alkyl groups?

A

Each alkyl group pushes electrons towards the C⁺ charge of carbocation. The positive charge of the carbocation is spread over the alkyl groups. More alkyl groups mean the positive charge is spread out more so the carbocation is more stable.

30
Q

What is a monomer?

A

Small molecule that combines to make polymers

31
Q

What is a polymer?

A

Long molecular chain made from repeating units

32
Q

What is a repeating unit?

A

An arrangement of atoms that occurs in a structure repeatedly.

33
Q

General equation for addition polymerisation?

A

W X [ W X ]
\ / [ | | ]
n C=C -> —[—C—C—]—
/ \ [ | | ]
Y Z [ Y Z ] n

34
Q

What is poly(ethene) also known as and what is it used for?

A

Polythene, used for plastic bags and bottles

35
Q

What is the equation for the formation of poly(ethene)?

A

H H [ H H ]
\ / [ | | ]
n C=C -> —[—C—C—]—
/ \ [ | | ]
H H [ H H ] n

36
Q

What is poly(propene) also known as and what are its uses?

A

Polypropene, used for plastic bowls, ropes and crates.

37
Q

What is the equation for the formation of poly(propene)?

A

H CH₃ [ H CH₃ ]
\ / [ | | ]
n C=C -> —[—C—C—]—
/ \ [ | | ]
H H [ H H ] n

38
Q

What is poly(chloroethene) also known as and what are its uses?

A

PVC, used for electrical cable insulation

39
Q

What is the equation for the formation of poly(chloroethene)?

A

H Cl [ H Cl ]
\ / [ | | ]
n C=C -> —[—C—C—]—
/ \ [ | | ]
H H [ H H ] n

40
Q

What is poly(phenylethene) also known as and what are its uses?

A

Polystyrene, used in packaging

41
Q

What is the equation for the formation of poly(phenylethene)?

A

H H [ H H ]
\ / [ | | ]
n C=C -> —[—C—C—]—
/ \ [ | | ]
H C₆H₆ [ H C₆H₆ ] n

(C₆H₆ is benzene)

42
Q

How are polymers disposed of?

A
  • Landfill
  • Combustion (incineration)
  • Reuse
  • Sorted and recycled
  • Use as organic feedstock
43
Q

Why might the landfill be used?

A

To dispose of plastics that are:

  • Too difficult to recycle
  • Too difficult to separate from other materials
  • Don’t have enough plastic to extract to make it economically viable
44
Q

What are issues with disposing of plastics in the landfill?

A
  • Not sustainable as large amounts of land are needed
  • Becoming increasingly expensive to use land for waste disposal
  • We need to reduce our reliance on landfill
45
Q

What ways are scientists trying to reduce environmental impacts of disposing of plastics?

A
  • develop photodegradable polymers
  • develop biodegradable polymers
  • develop techniques for cracking polymers
  • develop ways of making polymers from plant-based substances
  • design processes with high atom economy
  • develop ways of sorting AND recycling
46
Q

How can waste polymers be processed usefully?

A
  • Combustion for energy regeneration
  • Sorted and recycled
  • Sort, melt and remould
  • Cracking
  • Organic feedstock
47
Q

What is the issue when disposing of halogenated plastics and how is this combated?

A
  • Produces toxic HCl gas when burnt (as well as other pollutants like dioxins)
  • Flue gas scrubbers used to neutralise acidic gases

chemistry (1 decks)

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