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Chemistry > Organic - Alkenes > Flashcards

Organic - Alkenes Flashcards

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1
Q

What are alkenes?

A

Alkenes are unsaturated hydrocarbons. They are made of carbon and hydrogen only and have one or more carbon-carbon double bonds. This means that alkenes have fewer than the maximum possible number of hydrogen atoms.

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2
Q

Why does the double bond make alkenes more reactive than alkanes?

A

The double bond makes them more reactive than alkanes because of the higher concentration of electrons (high electron density) between the two carbon atoms.

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3
Q

What can ethene be used for?

A

Ethene, the simplest alkene, is the starting material for a large range of products, including polymers such as polythene, PVC, polystyrene, and terylene fabric, as well as products like antifreeze and paints.

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4
Q

When are alkenes produced in large quantities?

A

when crude oil is thermally cracked

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5
Q

What is the general formula for alkenes?

A

The homologous series of alkenes with one double bond has the general formula CnH2n.

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6
Q

How do you name alkenes?

A

There cannot be a C=C bond if there is only one carbon. So, the simplest alkene is ethene, CH2=CH2 followed by propene, CH3CH=CH2.

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7
Q

What is the shape of alkenes?

A

Ethene is a planar (flat) molecule. This makes the angles between each bend roughly 120 degrees.

Unlike the C-C bonds in alkanes, there is no rotation about the double bond. This is because of the make-up of a double bond. Any molecules in which a hydrogen atom in ethene is replaced by another atom or group will have the same flat shape around the carbon-carbon double bond.

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8
Q

Why can’t a double bond rotate?

A

As well as a normal C-C single bond, there is a p-orbital (which contains a single electron) on each carbon. These two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond. This is called a π orbital and its presence means the bond cannot rotate. This is sometimes called restricted rotation.

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9
Q

Why is the H-C-H angle slightly less than 120 degrees?

A

Because the group of four electrons in the C=C double bond repels more strongly than the groups of two in the C-H single bonds.

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10
Q

What are the two covalent bonds between the two carbon atoms?

A

Bond 1 = σ sigma covalent bond (electrons are between the two carbon atoms).

Bond 2 = π pi covalent bond (electrons above and below the plane of the 6 central atoms - formed from overlap of p orbitals on the carbon atoms).

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11
Q

What are sigma bonds?

A

A covalent bond resulting from the formation of a molecular orbital by the end-to-end overlap of atomic orbitals.

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12
Q

What are pi bonds?

A

A covalent bond resulting from the formation of a molecular orbital by side-to-side overlap of atomic orbitals along a plane perpendicular to a line connecting the nuclei of the atoms.

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13
Q

Which type of covalent bond is stronger?

A

Sigma bonds are the strongest type of covalent bonds due to the direct overlap of orbitals, and the electrons in these bonds are sometimes referred to as sigma electrons.

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14
Q

What are isomers?

A

Alkenes with more than three carbons can form different types of isomers and they are named according to the IUPAC system, using the suffix -ene to indicate a double bond.

As well as chain isomers like those found in alkanes, alkenes can form two types of isomer that involve the double bond:

  • position isomers
  • geometrical isomers
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15
Q

What are the two types of isomerism?

A
  • structural isomerism

- stereoisomerism

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16
Q

What is structural isomerism?

A

Molecules with the same molecular formula but a different structural formula.

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17
Q

What is stereoisomerism?

A

Molecules with the same molecular and structural formulas but a different arrangement of the atoms in space.

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18
Q

Give three examples of structural isomerism.

A
  1. Chain Isomers - caused by having a different carbon chain.
  2. Position Isomers - caused by the functional group being in a different position.
  3. Functional Group Isomers - caused by having a different functional group.
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19
Q

Give two examples of stereoisomerism.

A
  1. Geometric Isomers - caused by molecules with a C=C bond with two different groups attached to each C of the C=C bond.
  2. Optical Isomers - caused by carbon atoms having four different groups attached, leading to molecules that are non superimposable mirror images of each other.
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20
Q

What are position isomers?

A

These are isomers with the double bond in different positions, that is, between a pair of adjacent carbon atoms in different positions in the carbon chain.

The longer the carbon chain, the more possibilities there will be and therefore the greater the number of isomers.

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21
Q

What are geometrical isomers?

A

Geometrical isomerism is a form of stereoisomerism. The two stereoisomers have the same structural formula but the bonds are arranged differently in space. It occurs only around C=C double bonds.

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22
Q

What would you have to do in order to rotate around the C=C double bond?

A

break it

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23
Q

What are E-Z isomers?

A

If both carbons of the C=C have two different groups attached, the molecule has E-Z isomers.

The Cahn-Ingold-Prelog (CIP) priority rules are used to determine which is the E and which is the Z isomer.

  • E = entgegan (higher priority OPPOSITE)
  • Z = zusammen (highest priority TOGETHER)
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24
Q

What determines the priority of the group?

A

Whether a compound is an E or Z isomer depends on the priority of groups attached to C=C. The higher the ATOMIC number of the group attached to the carbon, the higher the priority. If they are the same, look at the atoms attached to those atoms.

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25
Q

What did E-Z isomerism used to be known as?

A

E-Z isomerism used to be known as cis-trans isomerism and the prefixes cis- and trans- were used inside of Z- and E- respectively.

However, a disadvantage of the older notation was that it did not work when there were more than two different substituents around a double bond. To give these two isomers different and unambiguous names, the E-Z notation is used.

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26
Q

What are the physical properties of alkenes?

A

The double bond does not greatly affect properties such as boiling and melting points. van der Waals forces are the only intermolecular forces that act between the alkene molecules. This means that the physical properties of alkenes are very similar to those of the alkanes. The melting and boiling points increase with the number of carbon atoms present. Alkenes are not soluble in water.

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27
Q

What is an electrophile?

A

an electron pair acceptor

28
Q

How do alkenes react?

A

The double bond makes a big difference to the reactivity of alkenes compared with alkanes. The bond enthalpy for C-C is lower than the bond enthalpy for C=C, so you would expect that alkenes would be less reactive that alkanes. In fact alkenes are more reactive than alkanes.

The C=C forms an electron-rich area in the molecule, which can easily be attacked by positively charged reagents. These reagents are called electrophiles (electron liking).

29
Q

Give an example of a good electrophile?

A

H+ ion

30
Q

What reactions can alkenes undergo?

A

As alkenes are unsaturated, they can undergo addition reactions.

31
Q

What are most of the reactions of alkenes?

A

electrophilic additions

32
Q

What makes alkenes susceptible to attack from electrophiles?

A

Alkenes are unsaturated hydrocarbons meaning they contain a carbon-carbon double bond. This is an area of high electron density making it susceptible to attack from electrophiles. Electrophiles are attracted to it and can form a bond by using two of the four electrons in the carbon-carbon double bond (of the four electrons, the two that are in a pi-bond).

33
Q

How can you test for an alkene?

A

Bromine water is used to identify this double bond and other unsaturated compounds. It turns the solution from orange-brown to colourless if a double bond is present in the substance.

34
Q

What will alkenes burn in?

A

Alkenes will burn in air. However, they are not used as fuels. This is because their reactivity makes them very useful for other purposes.

35
Q

What is the electrophilic addition reaction mechanism?

A
  1. The electrophile is attracted to the double bond (area of high electron density).
  2. Electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged ion or have a positively charged area.
  3. A positive ion (a carbocation) is formed.
  4. A negatively charged ion forms a bond with the carbocation.
36
Q

What is the main product formed by?

A

During the mechanism, a carbocation is formed. Where more than one carbocation can be formed, the main product will be formed from the more stable carbocation. In general, tertiary carbocations (with 3 R groups) are more stable than primary carbocations (with 1 R group and 2 H groups).

37
Q

How do alkyl groups form a major product?

A

Alkyl groups have a positive inductive effect and push electron density towards the positively charged carbon atom. This slightly reduces its reactivity, so its concentration builds up. Therefore it ends up reacting with the X- to form the major product.

38
Q

What is the reaction of alkenes with hydrogen halides?

A

Hydrogen halides, HCl, HBr and HI, add across the double bond to form a halogenoalkane.

  • Bromine is more electronegative than hydrogen, so the hydrogen bromide molecule is polar, Hbeta+-Brbeta-.
  • The electrophile is the Hbeta+.
  • The Hbeta+ of HBr is attracted to the C=C bond because of the double bond’s high electron density.
  • One of the pairs of electrons from the C=C forms a bond with the Hbeta+ to form a positive ion (called a carbocation), whilst at the same time the electrons in the Hbeta+-Brbeta- bond are drawn towards the Brbeta-.
  • The bond in hydrogen bromide breaks heterolytically. Both electrons from the shared pair in the bond go to the bromine atom because it is more electronegative than hydrogen leaving a Br- ion.
  • The Br- ion attached to the positively charged carbon of the carbocation forming a bond with one of its electron pairs.
39
Q

What does heterolytic bond breaking mean?

A

Heterolytic bond breaking means that when a covalent bond breaks, both the electrons go to one of the atoms involved in the bond and none to the other. This results in the formation of a negative ion and a positive ion.

40
Q

Explain asymmetric alkenes in regards to the hydrogen halide reaction with alkenes.

A

When hydrogen bromide adds to ethene, bromoethene is the only possible product. However, when the double bond is not exactly in the middle of the chain, there are two possible products - the bromine of the hydrogen bromide could bond to either of the carbon atoms of the double bond.

41
Q

What is Markovnikov’s rule?

A

There is a simple way to work out the product. When hydrogen halides add on to alkenes, the hydrogen adds on to the carbon atom which already has the most hydrogens.

42
Q

How can you explain why although there are asymmetrical alkenes, the product is almost always one variation?

A

Alkyl groups have a tendency to release electrons. This is known as a positive inductive effect and is sometimes represented by an arrow along their bonds to show the direction of the release.

This electron-releasing effect tends to stabilise the positive charge of the intermediate carbocation. The more alkyl groups there are attached to the positively charged carbon atom, the more stable the carbocation is. So, a positively charged carbon atom which has three alkyl groups (called a tertiary carbocation) is more stable than one with two alkyl groups (a secondary carbocation) which is more stable than one with just one (a primary carbocation).

The product will tend to come from the more stable carbocation.

43
Q

What conditions are needed for a reaction with hydrogen halides?

A

no specific condition

44
Q

What is the reaction of alkenes with halogens?

A

Alkenes react rapidly with chlorine gas, or with solutions of bromine and iodine in an organic solvent, to give dihalogenoalkanes.

The halogen atoms add across the double bond. In this case, the halogen molecules act as electrophiles.

45
Q

How do alkenes react with halogens?

A
  • At any instant, a bromine (or any other halogen) molecule is likely to have an instantaneous dipole, Brbeta+-Brbeta-. (An instant later, the dipole could be reversed Brbeta–Brbeta+.) The beta+ end of this dipole is attracted to the electron-rich double bond in the alkene - the bromine molecule has become an electrophile.
  • The electrons in the double bond are attracted to the Brbeta+. They repel the electrons in the Br-Br bond and this strengthens the dipole of the bromine molecule.
  • Two of the electrons from the double bond forms a bond with the Brbeta+ and the other bromine atom becomes a Br- ion. This leaves a carbocation, in which the carbon atom that is not bonded to the bromine has the positive charge.
  • The Br- ion now forms a bond with the carbocation.
46
Q

How else is this dipole induced?

A

This dipole is also induced when a bromine molecule collides with the electron-rich double bond.

47
Q

What two steps does the reaction between alkenes and halogens take place in?

A
  1. formation of the carbocation by electrophilic addition

2. rapid reaction with a negative ion

48
Q

What conditions are needed for the reaction between alkenes and a halogen?

A

aqueous (i.e. bromine water)

49
Q

What is the test for a double bond?

A

This addition reaction is used to test for a carbon-carbon double bond. When a few drops of bromine solution, sometimes called bromine water (which is reddish-brown) are added to an alkene, the solution decolourises because the products are colourless.

50
Q

What is the reaction of alkenes with concentrated sulphuric acid?

A

Concentrated sulphuric acid also adds across the double bond. The reaction occurs at room temperature and is exothermic.

The electrophile is a partially positively charged hydrogen atom in the sulphuric acid molecule. The carbocation which forms then reacts rapidly with the negatively charged hydrogensulphate ion.

When water is added to the product, an alcohol is formed and sulphuric acid reforms. The overall effect is to add water across the double bond and the sulphuric acid is a catalyst for the process.

51
Q

Explain asymmetric alkenes in regards to the sulphuric acid reaction with alkenes.

A

In an asymmetrical alkene, the carbocation is exactly the same as that found in the reaction with hydrogen bromide. This means that you can predict the products by looking at the relative stability of the possible carbocations that could form.

52
Q

What conditions are needed for the reaction between alkenes and sulphuric acid?

A

concentrated H2SO4, cold (typically at room temperature)

53
Q

What is the reaction of alkenes and water?

A

Water also adds on across the double bond in alkenes. The reaction is used industrially to make alcohols and is carried out with steam, at a suitable temperature and pressure, using an acid catalyst such as phosphoric acid.

54
Q

What are polymers?

A

Polymers are very large molecules that are built up from small molecules, called monomers. The process in which monomers join together is called polymerisation. There are two types of polymers: addition (no other products formed) and condensation (a small molecule such as water is lost). They occur naturally everywhere: starch, proteins, cellulose and DNA are all polymers.

One way of classifying polymers is by the type of reaction by which they are made.

55
Q

What are addition polymers?

A

Addition polymers are made from a monomer or monomers with a carbon-carbon double bond (alkenes). The monomer has the general formula CH2CHR.

56
Q

What happens when monomers polymerise?

A

When the monomers polymerise, the double bond opens and the monomers bond together to form a backbone of carbon atoms.

57
Q

How do you identify the addition polymer formed from the monomer?

A

An addition polymer is formed from monomers with carbon-carbon double bonds.

There is usually only one monomer (though it is possible to have more), and the double bond opens to form a single bond. This will give the repeat unit for the polymer.

58
Q

How do you identify the monomer used to make an addition polymer?

A

An addition polymer must have a backbone of carbon atoms and the monomer must contain at least two carbons, so that there can be a carbon-carbon double bond.

59
Q

How are plastics modified?

A

The properties of polymers materials can be considerably modified by the use of additives such as plasticisers. These are small molecules that get between the polymer chains forcing them apart and allowing them to slide across each other.

For example, PVC is rigid enough for use as drainpipes, but with addition of plasticise, it becomes flexible enough for making aprons.

60
Q

Why are polyalkenes chemically inert?

A

Addition polymers are polyalkenes. The molecules are saturated, as all of the bonds holding the carbon chain together are single. They contain multiple strong, non-polar covalent bonds. This makes them generally unreactive and chemically inert.

61
Q

Why are polyalkenes not biodegradable?

A

Polyalkenes, in spite of their name, have a backbone which is a long chain saturated alkane molecule. Alkanes have strong non-polar C-C and C-H bonds. So, they are very unreactive molecules, which is a useful property in many ways. However, this does mean that they are not attacked by biological agents - like enzymes - and so they are not biodegradable. This is an increasing problem in today’s world, where waste disposal is becoming more and more difficult.

62
Q

How can you reduce the amount of plastic? (solution to pollution by plastics)

A

it can be reused or recycled

63
Q

What is mechanical recycling?

A

The simplest form of recycling is called mechanical recycling. The first step is to separate the different types of plastics. (Plastic containers are now collected in recycling facilities for this purpose.) The plastics are then washed and once they are sorted, they may be group up into small pellets. These can be melted and remoulded. For example, recycled soft drinks bottles made from PET (polyethylene terephthalate) are used to make fleece clothes.

64
Q

What is feedstock recycling?

A

Here, the plastics are heated to a temperature that will break the polymer bonds and produce monomers. These can then be used to make new plastics.

There are problems with recycling. Poly(propene), for example, is a thermoplastic polymer. This means that it will soften when heated so it can be melted and re-used. However, this can only be done a limited number of times because at each heating, some of the chains break and become shorter thus degrading the plastic’s properties.

65
Q

What are plasticisers?

A

The properties of polymers can be tailored to make them suitable for a variety of application by the use of various additives. Plasticisers are small molecules that get in between the polymer chains, pushing them apart and making them more flexible. This reduces the strength of the intermolecular forces, making it easier for the chains to slide over each other. This is how poly(chlorethene) also called polyvinyl chloride (PVC) or just vinyl can be made rigid enough for use as drainpipes and flexible enough for plastic aprons.

66
Q

What reaction conditions are needed for the production of polymer chains?

A

The reaction conditions used in the production of these polymer chains can be altered to give the plastics produced different properties.

High pressures and temperatures produce branched chain polymers with weak intermolecular forces. Whereas lower pressures and temperatures produce straight chain polymers with strong intermolecular forces.

67
Q

What are the properties of addition polymers?

A
  • Polymers are saturated (single bonds between carbon atoms in the main chain).
  • The carbon chain back bone is usually non-polar.
  • This makes them unreactive (chemically inert), so they are very hard to break down.
  • Strong, covalent bonds within the polymer molecules, weak intermolecular forces (usually van der Waals) between the polymer chains.
  • The stronger the intermolecular forces, the higher the melting point.
  • Most polymers contain a mixture of chains of various lengths so melt over a range of temperatures.

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