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Chem Mock > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

What type of bonds do alkenes have

A

double carbon bond

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2
Q

What is the formula

A

CnH2n

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3
Q

Are they saturated or unsaturated

A

unsaturated hydrocarbons

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4
Q

what is the functional group

A

double carbon bond, which is the functional group and is what allows alkenes to react in ways that alkanes cannot

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5
Q

when should you state the position of the double bond

A

In molecules with a straight chain of 4 or more carbon atoms

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6
Q

how is a π bond formed

A

the sideways overlap of adjacent p orbitals
The two lobes that make up the π bond lie above and below the plane of the σ bond. Resulting in no free rotation of the groups about the C atoms

single π bond is drawn as two electron clouds, which represent one bond containing two electrons

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7
Q

What are sigma bonds

A

end to end overlap of atomic orbitals
The electron density in a σ bond is symmetrical between the nuclei of the atoms forming the bond
The pair of electrons is found between the nuclei of the two atoms

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8
Q

explain the sigma and pi bonds in Ethene

A

Each carbon atom uses three of its four electrons to form σ bonds
Two σ bonds are formed with the hydrogen atoms
One σ bond is formed with the other carbon atom
The fourth electron from each carbon atom occupies a p orbital which overlaps sideways with another p orbital on the other carbon atom to form a π bond
This means that the C-C is a double bond: one σ and one π bond

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9
Q

what area is of high electron density in alkenes

A

double bond as four electrons found here making it susceptible to attack by electrophiles

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10
Q

what is electrophilic addition

A

addition of an electrophile to a double bond. The
C-C double bond is broken, and a new single bond is formed from each of the two carbon atoms

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11
Q

what are examples of electrophilic addition

A

Hydrogen (also known as hydrogenation reaction)
Steam (H2O (g))
Hydrogen halide (HX)
Halogen

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12
Q

What is the hydrogenation reaction

A

alkene + H2 –> Alkane
(Nickel catalyst)

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13
Q

what is the reaction with steam

A

alkene + H2O –> alcohol
(H3PO4)

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14
Q

reaction with hydrogen halide

A

alkene + HBr –> Halogenoalkane

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15
Q

reaction with halogen

A

alkene + halogen –> halogenoalkane

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16
Q

how is margarine formed

A

Naturally occurring vegetable oils are unsaturated and contain C=C double bonds

When these bonds react with hydrogen, some of the C=C double bonds become C-C single bonds

This process changes the properties of vegetable oil and converts it into a solid: margarine

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17
Q

How can alkenes be oxidised

A

by acidified potassium manganate(VII) (KMnO4) which is a very powerful oxidising agent

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18
Q

Explain the process of oxidation of alkenes

A

When shaken with cold dilute KMnO4 the pale purple solution turns colourless and the product is a diol. distinguish alkanes from alkenes ( alkanes do not have double bonds and so are not oxidised in this way)

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19
Q

equation for oxidation of alkene and where two OH are come from

A

The potassium manganate provides an oxygen atom (oxidation)
Then water in the solution provides another oxygen atom and two hydrogen atoms, so there is addition of two OH groups across the double bond

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20
Q

what is heterolytic fission

A

breaking a covalent bond in such a way that the more electronegative atom takes both the electrons from the bond to form a negative ion and leaving behind a positive ion

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21
Q

Are hydrogen halides polar

A

are polar as the hydrogen and halogen atoms have different electronegativities

The bromine atom has a stronger pull on the electrons in the H-Br bond
As a result of this, the Br atom has a partial negative and the H atom a partial positive charge

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22
Q

What becomes the electrophile from HBr

A

Hydrogen as it is electron deficienct and accepts a pair of electrons from the C-C bond in the alkene

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23
Q

How does the H-Br bond break

A

breaks heterolytically, forming a Br- ion
This result in the formation of a highly reactive carbocation which reacts with the Br- (nucleophile)

24
Q

How can you identify which product is major or minor

A

look at the carbon cation whichever one is bonded to most carbons is more stable so more likely to form hence the major product from the nucleophilic attack

25
Q

are halogens polar

A

are a non-polar molecules as both atoms have similar electronegativities and therefore equally share the electrons in the covalent bond

26
Q

what happens when bromine gets close to alkene double bond

A

the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom
As a result of this, the closest Br atom to the double bond is slightly positive and the further Br atom is slightly negatively charged creates induced dipole

27
Q

What is the saturation test

A

Halogens can be used to test if a molecule is unsaturated.
Br2(aq) is an orange or yellow solution, called bromine water and this is the halogen most commonly used
The unknown compound is shaken with the bromine water
If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

28
Q

carbon cation

A

a positively charged carbon atom

29
Q

What is addition polymerisation

A

the reaction in which many monomers containing at least one C=C double bond form long chains of polymers as the only product

30
Q

how does addition polymerisation work

A

Just like in other addition reactions of alkenes, the π-bond in each C-C bond breaks and then the monomers link together to form new C-C single bonds

31
Q

what is a polymer

A

long-chain molecule that is made up of many repeating units

32
Q

what is a monomers

A

The small, reactive molecules that join together to form the polymer

33
Q

why are polymers unreactive

A

The addition polymers made from alkenes are saturated compounds because they do not contain double bonds and the main carbon chain is non-polar

34
Q

what is a repeating unit

A

portion of the whole molecule that repeats itself several times. A polymer consists of lots of repeating units joined together. The repeating unit has a similar structure to the monomer, except it has the double bond opened up.

35
Q

describe the reactivity and non-biodegrade polymers

A

The low reactivity of many polymers makes them ideal for certain uses, e.g. food packaging, but at the same time creates problems with their disposal as a lot of polymers are non-biodegradable

36
Q

explain how recycling of polymers work

A

Polymer recycling reduces the amount of waste that it going to landfill sites

Newer landfill sites can have a recycling point where the new waste is brought before going to into the actual landfill - this is in an effort to reduce the amount of polymers (and other recyclable materials) unnecessarily going into the waste site

37
Q

what does recycling polymers do

A

reduce the use of finite resources
Lots of polymers are made from the products of cracking crude oil and it’s fractions

38
Q

what is a disadvantage of recycling polymers

A

Recycling polymers is a time-consuming process as they have to be sorted into the different categories

39
Q

why can polymers be incinerated

A

Since they have a large amount of energy stored within the polymer chains.

40
Q

what can incineration do

A

This process can then be used to boil water and use the water vapour to turn turbines inside a power station, in a similar fashion to coal-fired power stations

41
Q

does incineration cause environmental pollution

A

as the carbon within the polymer can be released as carbon dioxide contributing to global warming
Other toxic waste products include hydrogen chloride and other chlorinated molecules from the combustion of PVC

42
Q

What is feedstock recycling

A

where waste polymers are broken down, by chemical and thermal processes, into monomers, gases and oils

43
Q

What are products used for

A

the raw materials in the production of new polymers and other organic chemicals

44
Q

benefit of feedstock recycling

A

compared to other methods of polymer disposal, is that it works with unsorted and unwashed polymers

45
Q

what are some aims when designing a sustainable polymer manufacturing process

A

Use chemicals that are as safe and environmentally friendly as possible

Use renewable feedstock chemicals where possible

Reduce the energy requirements for the reaction as well as increase the energy efficiency, which has both an environmental and financial bonus

Improve atom economy

46
Q

how can you remove toxic waste products

A

The waste gase from the incinerator are scrubbed/reacted with a base or carbonate
The base reacts with the acidic HCl gas, neutralising it
CaO (s) + 2HCl (aq)→CaCl2 (aq) + H2O (l)

47
Q

how can biodegradable polymers be broken down

A

broken down over time by microorganisms
Common products from this process include carbon dioxide, water and other organic compounds

48
Q

uses of waste plastic

A

incineration to release energy

recycling

feedstock recycling

49
Q

what is a stereoisomer

A

compounds which have the same structural formula but their atoms are arranged differently in space.

50
Q

what is structural isomer

A

molecules that have the same molecular formula, but have a different arrangement of atoms

51
Q

why is incineration not a suitable method of disposal

A

because it produces HCl which is corrosive and causes acid rain

52
Q

what is a dipole

A

when electrons are shared unequally between atoms in the same molecule due to a high difference in the electronegativity of the atoms involved.

53
Q

why cant LiAlH4 be used to reduce alkenes

A

because the hydride ion will be repelled by regions of high electron density

54
Q

how do you spot which carbon is carbocation

A

the tertiary one as it is more stable and more positively inductive

55
Q
A

Chem Mock (34 decks)

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