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A-Level Chemistry Paper 2- Core Organic Chemistry > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

what are alkenes?

A

hydrocarbons with the general formula (CnH2n)

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2
Q

how do alkenes differ from the alkanes?

A

they have a to carbon=carbon double bond
This is a region of high electron density so they will react with electrophiles (electron electron pair acceptor)

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3
Q

What does the bonding with alkenes contain?

A

-A double bond which is made up of two distinct bonding regions the sigma bond and the pi bond
-The electrons are localised between two carbon atoms

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4
Q

what are hydrogens attached to carbon bonds by?

A

Sigma bonds

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5
Q

what does the carbon to carbon sigma bond have?

A

-Electrons directly between the two carbon nuclei

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6
Q

how was the pi bond formed?

A

By the sideways overlap of adjacent P orbitals above and below the bonding C atoms

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7
Q

Where is where is the pi bond?

A

Above and below the plane of the molecule

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8
Q

what is a carbon carbon double bond compromised of?

A

A sigma bond surrounded by a pi bond

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9
Q

what is the geometry (shape) and bond angle around the carbon atoms?

A

-trigonal planar
-the bond angle is 120° (three bonding regions and zero lone pairs)

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10
Q

what is structural isomerism?

A

molecules with the same molecular formula but different structural formula

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11
Q

what are the four structural isomers of C4 H8?

A

-but-1-ene
-but-2-ene
-methylpropene
-cyclobutane

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12
Q

what is what is stereoisomerism?

A

-stereoisomers are compounds with the same structural formula, but a different arrangement of atoms in space

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13
Q

what is E/Z isomerism (geometric isomerism)?

A

-arises from having two different groups on each carbon atom of the carbon-carbon double bond
-and the restricted rotation about the carbon-carbon double bond

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14
Q

why do we have E/Z isomerism?

A

-because of the restrict rotation around c=c

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15
Q

when do we have E/Z isomerism?

A

must have c=c, each atom/group bonded or each of the carbons in c=c must be different

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16
Q

how do you name alkenes?

A

-The ending for all alkenes is -ene
-for a carbon chain length longer than three carbon atoms, you must state the position of the double bond

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17
Q

what is an electrophile?

A

-an electron pair acceptor
-They are positive charge carriers (cation or polar)

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18
Q

what are more reactive alkanes or alkenes?

A

alkenes

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19
Q

What is the bond enthalpy of a pi bond compared to a sigma bond?

A

A pi bond has a lower bond enthalpy compared to a sigma bond

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20
Q

where do alkenes contain a high electron density?

A

-within the double bond
-The electrons in the double bond attract electrophiles

21
Q

what are the reactions of Alkenes?

A

-addition of hydrogen, H2- hydrogenation
-Addition of halogen, X2- halogenation
-Addition of hydrogen halide, HX- halogenation
-Addition of water, H2O- hydration
-Addition of polymerisation

22
Q

what are the reagents and conditions of hydrogenation?

A

-Hydrogen (H2)
-Nickel (Ni) catalyst

23
Q

what is the importance of hydrogenation?

A

-the unsaturated alkene becomes saturated
-used in the manufacture of margarine (hydrogenating vegetable oils)

24
Q

What are the reagents and conditions of halogenation?

A

-Cl2, or Br2 or I2
-room temp

25
Q

what is the addition bromine (halogenation)

A

the Laboratory test for the presence of double bonds

26
Q

what is that addition of iodine used to do? (halogenation)

A

-Test for unsaturation in vegetable oils

27
Q

what is the chemical test for alkenes?

A

-add unsaturated compound into bromine
-shake
-bromine water turns colourless if double bonds are present

28
Q

What are the reagents and conditions for addition of hydrogen halides?

A

-HCl or Hbr or HI
-Room Temp

29
Q

why will alkenes give rise to 2 products when reacting with hydrogen halides and other non-symmetrical molecules?

A

-Alkenes are unsymmetrical
-For example propane reacts with hydrogen bromide to give to isomeric products

30
Q

How does the reactivity of the hydrogen halide increase?

A

-From HF to HI, following the order of the decreasing bond energy

31
Q

how does the reactivity of the haloalkanes increase?

A

Down the group as the strength of the hydrogen halide bond this weaker

32
Q

what are the reagents and conditions for hydration?

A

-steam (H2O (g)); (Temp> 100°C)
-Phosphoric acid (H3PO4) catalyst

33
Q

What is the importance of hydration?

A

-addition of steam across a double bond is used in the industrial manufacture of alcohols

34
Q

What will the major product be in the mechanism for an unsymmetrical electrophile?

A

The major product will be the one which results from the most stable carbocation

35
Q

what is a carbocation?

A

Molecular ion in which a caron atom bears three bonds and a positive charge. There are three classifications of carbocations.

36
Q

What are the classifications of carbocations?

A

-Primary (carbocation bonded to 1 C atom)
-Secondary (carbocation bonded to 2 C atoms)
Tertiary (carbocation bonded to 3 C atoms)

37
Q

why is there an increase in stability in the three classifications of carbocations?

A

-due to the presence of an increasing number in alkyl groups

38
Q

what does the alkyl group do in terms of stability?

A

The alkyl group pushes the pair of electrons within the covalent bond towards the carbocation, making it ‘less’ positive, and more stable

39
Q

what is the major product dependent on in an electrophilic addition mechanism with an unsymmetrical, alkene and unsymmetrical small molecule?

A

The stability of the carbocation intermediates

40
Q

what is the major product formed from?

A

The most stable carbocation intermediate in the mechanism

41
Q

what is the minor product formed from?

A

The least stable carbocation intermediate in the mechanism

42
Q

what is the more stable a cation result in?

A

-The longer it exists in the reaction
-This means there is a greater chance of it reacting with bromide ion

43
Q

What are polymers?

A

Long-chained molecules made by the joining of many small chained molecules called monomers

44
Q

what undergoes addition polymerisation?

A

alkenes

45
Q

why do alkenes undergo addition polymerisation?

A

To produce saturated chains, containing no double bonds

46
Q

What is broken and made in addition polymerisation?

A

A pie bond is broken, and two new sigma bonds are made

47
Q

What are addition polymers made from?

A

One type of monomer only

48
Q

What molecular mass can polymers have?

A

mass of several thousand

A-Level Chemistry Paper 2- Core Organic Chemistry (5 decks)

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