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developing fuels > DF 6 - alkenes, versatile compounds > Flashcards

DF 6 - alkenes, versatile compounds Flashcards

specification reference - (b) (m) (o) (q)

1
Q

what is the difference between saturated and unsaturated

A

Saturated hydrocarbons contain only single covalent bonds while unsaturated hydrocarbons contain at least one or more double or triple carbon-carbon bonds.

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2
Q

which contain more saturated fats

A

animal fats such as butter and lard

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3
Q

which contain more unsaturated fats

A

plant derived fats and oils

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4
Q

why are saturated fats associated with heart disease

A

they were thought to cause a buildup of cholesterol in the arteries

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5
Q

which types of fats cause heart disease

A

trans fats

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6
Q

what does cis and trans refer to

A

the arrangement of the groups across the carbon=carbon double bond

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7
Q

what type of fats are naturally occuring unsaturated fats

A

cis fats

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8
Q

why aren’t cis fats used in the food industry

A

they often have melting points that are slighltly too low for use in the food industry

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9
Q

how did cis fats turn to trans fats

A

partial hydrogenation was carried out to react the fats with hydrogen in order to saturate some of the unsaturated bonds. in the process some of the cis compounds were turned to trans

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10
Q

why are trans fats being phased out

A

they are found to likely lead to a buildup of cholesterol

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11
Q

what is an alkene

A

unsaturated hydrocarbons with a double C=C bond.

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12
Q

what is the general formula of non cyclic alkenes

A

CnH2n (same as cycloalkanes but the double bond makes alkenes react very differently from cycloalkanes)

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13
Q

rules for naming alkenes

A

1) identify the longest carbon chain and use the appropiate prefix to the number of carbons
2) follow prefix ‘ene’ to indicate the compound is an alkene
3) for alkenes, where longest carbon chain is four carbon atomd or longer insert a number before ‘ene’ to indicate the location of the C=C double bond

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14
Q

why are alkenes more reactive than alkanes

A

because of the double C=C bond

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15
Q

which is stronger - a double bond or single bond

A

double bond

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16
Q

what is the single bond between two carbon atoms in alkanes called

A

a sigma bond σ

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17
Q

what is the double bond in an alkene made up of

A

a sigma bond σ and a pi bond 𝛑

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18
Q

how are σ bonds formed

A

by the head-on overlap of s orbitals. the first bond between two atoms in a molecule is always a sigma bond

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19
Q

how are the electrons arranged in a single bond

A

the two electrons are arrnaged between the atoms in an area of increased electron density

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20
Q

how are 𝛑 bonds formed

A

by the sideways overlap of p-orbitals. these bonds occur in addition to a sigma bond (double, triple bonds)

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21
Q

what is the bonding like in a
- single bond
- double bond
- triple bond

A

single bond - a sigma bond
double bond - a sigma bond and a pi bond
triple bond - a sigma bond and two pi bonds

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22
Q

which are stronger 𝛑 or σ bonds

A

σ bonds

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23
Q

what happens to the bonds when an alkene reacts

A

the 𝛑 bond breaks but the σ remains intact

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24
Q

what is an addition reaction

A

where two or more molecules react to form a single, larger molecule

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25
Q

what takes part in an addition reaction

A

alkenes take part in addition reactions. a small molecule is added across the double bond, causing the 𝛑 bond to be broken

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26
Q

what happens in an addition reaction

A

the 𝛑 bond breaks. two reactant molecules combine to form one product molecule. an unsaturated alkene forms a saturated molecule

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27
Q

does the saturated molecule have to be an alkane

A

no. its a molecule with no double bonds

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28
Q

give the equation for a basic addition reaction of an alkene

A

\ / X Y
C=C + X-Y —> | |
/ \ —C —–C—
| |

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29
Q

what is the addition of hydrogen also called

A

hydrogenation

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30
Q

what happens in hydrogenation

A

a mixture of hydrogen gas and a gaseous alkene is passed over a nickel catalyst and at a temperature of 150°C and a pressure of 5atm
the hydrogen adds across the double bond and an alkane is formed

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31
Q

example of hydrogenation

A

ethene + H2 —> ethane

32
Q

what is the addition of halogens called

A

halogenation

33
Q

what happens in halogenation

A

alkenes react rapidly with halogen such as chlorine and bromine at room temperature. the halogen adds across the double bond to give a halogenoalkane. (if its chlorine, its chlorination - bromine, then bromination)

34
Q

example of halogenation

A

ethene + Br2 —>
1,2 - dibromoethane

35
Q

how do we test if something is unsaturated

A

when bromine is added to a sample containing an alkene, it changes from orange to colourless. the colour change shows that the bromine has been added across the double bond. the reactant is used as a test for unsaturation and proves the presence of a double bond

36
Q

what happens in the addition of hydrogen halides

A

a hydrogen halide such as HCl, HBr and HI add across the double bond to form a halogenoalkane. happens at room temperature

37
Q

example of addition of hydrogen halides

A

ethene + HBr —> bromoethane
HBr is in solution in a polar solvent

38
Q

what is the addition of steam sometimes called

A

hydration of steam

39
Q

what happens in hydration of steam

A

steam and the gaseous alkene are to a high temperature and pressure in the presence of a phosphoric acid catalyst. 300°C and 60atm

40
Q

when is hydration of steam to alkenes used

A

the addition of steam to alkenes is one method of preparing alchohols. this reaction is widely used in manufacturing

41
Q

example of hydration of steam

A

ethene + steam (H2O) ———–
phosphoric acid catalyst (H3PO4)
———-> ethanol (alcohol)

42
Q

how else is ethanol made

A

in the lab ethene can be converted to ethanol by first adding concentrated sulfuric acid and then diluting with water

43
Q

what does the addition to unsymmetrical alkenes result in

A

forms a mixture of organic products

44
Q

example of addition to unsymmetrical alkenes

A

propene + hydrogenbromide
- forms a major product = 2-bromopropane
- forms a minor product = 1-bromopropane

45
Q

what do the major and minor products refer to

A

the relative amounts of the products formed, based on the stability of the carbocation intermediate during the reaction
the major product is formed in greater quantity
the minor product is formed in a smaller quantity

46
Q

what is Markownikov’s rule

A

in the addition of a hydrogen halide to an alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms and the halogen attaches to the carbon with fewer hydrogen atoms.

47
Q

what does electrophile mean

A

a positive ion or molecule with a partial positive charge that will be attracted to a negatively charged region and react by accepting a lone pair of electrons to form a covalent bond

48
Q

what is the charge of an electrophile

A

positive/neutral

49
Q

why does the region between two carbon atoms in ethene have a high density of negative charge

A

there are four electrons in the double bond of ethene giving it a higher negative charge

50
Q

what is attracted to the negative charge of the c=c double bond

A

positive ions or molecules with a partial positive charge on one of the atoms will be attracted to this negatively charged region

51
Q

how do positive ions react with the negatively charged region of the C=C bond

A

by accepting a pair of electrons from the C=C double bond

52
Q

what is the test for unsaturation

A

when ethene is bubbled through bromine, the red brown bromine becomes decolourised

53
Q

what is a reaction mechanism

A

logically deciding movement of electrons using ideas of bond polarity and charge

54
Q

what happens when bromine for e.g. approaches an alkene

A

bromine molecules become polarised as it approaches the alkene. the electrons in the bromine are repelled by the alkene reactions and are pushed back along the molecule.
the bromine atom nearest the alkene becomes slightly positively chargedd and the bromine atom further away from the alkene becomes slightly negatively charged.

55
Q

what happens to the slightly positive bromine atom

A

it behaves like an electrophile and reacts with the alkene double bond

56
Q

what happens to the other carbon atom after the bromine adds to the other one

A

it now only has six outer electrons and has become positively charged. it is a carbocation

57
Q

what is a carbocation

A

an ion with a positively charged carbon atom. they react very rapdily with anything that has electrons in it

58
Q

what happens to the carbocation (bromine)

A

a pair of electrons move from the bromide ion to the positively charged carbon to form a new carbon-bromine covalent bond

59
Q

what does electronegative mean

A

a measure of an atom’s ability to attract shared electrons to itself.

60
Q

which atoms are more electronegative

A

On the periodic table, electronegativity generally increases as you move from left to right across a period and decreases as you move down a group.

61
Q

out of these elements which is the least and most electronegative
HF
HCl
HBr
HI

A

HF slowest reaction/strong
most electronegative
HCl
HBr
HI fastest reaction/weak
least electronegative

62
Q

what does phosphoric acid look like

A

H
\
O
|
O - P - O
/ || \
H O H

63
Q

what makes phosphoric acid polar

A

it has electronegative elements

64
Q

which elements are δ− and which are δ+ in phosphoric acid

A

oxygen is δ-
hydrogen is δ+

65
Q

describe the conditions needed for hydration of alkenes such as ethene

A

requires phosphoric acid catalyst at temperature 300°C and 60atm and water in the form of steam

66
Q

what is stage 1 of the hydration of steam with ethene mechanism

A

stage 1 - pair of electrons in the 𝛑 bond of ethene are attracted to one of the δ+ hydrogen atoms in phosphoric acid.
- electrons in the pi bond form a covalent bond with the positive hydrogen atom. the covalent bond between hydrogen and oxygen breaks.
- pair of electrons move to oxygen atom

67
Q

what is heterolytic fission

A

when a covalent bond breaks, one bonding atom receives both electrons from the bonded pair.

68
Q

what is a carbocation intermediate

A

a positively charged intermediate on the carbon atom that has not yet formed a bond.

69
Q

what forms at the end of stage 1 of the hydration of steam

A

a carbocation intermediate with a positive carbon atom forms because of stage 1. we also have a dihydrogen phsophate ion with a negatively charged oxygen atom

70
Q

what is stage two of the hydration of steam with ethene mechanism

A

water in the form of steam reacts with the carbocation intermediate. the oxygen in a water molecule has two lone pairs of electrons. one lone pairs forms a covalent bond between the oxygen and the positive carbon in the carbocation intermediate. this leaves the oxygen to become positive as one of its lone pairs has formed a covalent bond

71
Q

what is stage 3 of the hydration of steam with ethene mechanism

A

the dihydrogen phosphate ion forms a covalent bond to a hydrogen (from the water molecule). at the same time, the covalent bond in the intermediate breaks by heterolytic fission. the pair of electrons move to the oxygen

72
Q

what is stage 4 of the hydration of steam with ethene mechanism

A

ethanol is formed and the phosphoric acid catalyst is regenerated.

73
Q

what experimental evidence supports the mechanism for electrophillic addition via a carbocation

A

if hex-1-ene reacts with bromin in the presence of chloride ions, two products are formed: 1,2-dibromohexane and 1-bromo-2-chlorohexane
1,2-dichlorohexane is not formed as the electrophile must attack first then the anions (chloride ions)

74
Q

what does the test for an alkene use instead of pure bromine

A

bromine water

75
Q

what does bromine water do to the reaction with an alkene

A

water molecules have lone pairs of electrons and act as nucleophile in competition with bromine ions. this forms a bromoalcohol

76
Q

what would happen if the bromine water was dilute

A

there would be too many water molecules than bromine ions present and the bromoalcohol will be the main product of the reaction. the bromine water is still decolourised

developing fuels (11 decks)

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