Alkenes, nomenclature, physical and chemical properties
методическая разработка по химии (10 класс) по теме

UNSATURATED HYDROCARBONS

Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. Those with at least one double bond are called alkenes and those with at least one triple bond are called alkynes. Alkenes with two double bonds are called dienes.

ALKENES

Alkenes – are acyclic hydrocarbons containing one double bond between carbon atoms and have general formula CnH2n. Carbon atoms having double bond make sp2 hybridization.

They are also known as olefins, because they react with chlorine to form an oil-like liquid.

R = alkyl group or H

The chemical behavior of alkenes is largely determined by the C = C functional group.

         Table: First nine members of the alkenes

Non IUPAC names of some alkenes

NOMENCLATURE OF ALKENES

Many of the same rules for alkanes apply to alkenes

Name the parent hydrocarbon by locating the longest carbon chain that contains the double bond and name it according to the number of carbons with the suffix –ene.

a. Number the carbons of the parent chain so the double bond carbon have the  

        lowest possible numbers.

    b. If the double bond is equidistant from each end, number so the first substituent

        has the lowest number.

Write out the full name, numbering the substituents according to their position in the chain and list them in alphabetical order.

Indicate the double bond by the number of the first alkene carbon.

If more than one double bond is present, indicate their position by using the number of the first carbon of each double bond and use the suffix –diene (for 2 double bonds), -triene (for 3 double bonds), etc.

Cycloalkenes are named in a similar way. Number the cycloalkene so the double bond carbons get numbers 1 and 2, and the first substituent is the lowest possible number.

Alkenes as substituents

Example:

Isomers of alkenes

Alkenes show four types of isomerism, which are described below.

Chain isomerism

Position isomerism

Geometrical isomerism

Functional isomerism.

Chain isomerism

This is due to the difference in the structure of the carbon chain. For example, butene-1 and 2-methylpropene are chain isomers.

Position isomerism

Position isomerism is the differences that arise in the position of the double bond in the same chain of alkenes. Butene-1 is a position isomer of butene-2 because these differ only in the position of the double bond.

CH3-CH2-CH=CH2  1- butene          CH3-CH=CH-CH3      2-butene

Geometrical isomerism

Two carbon atoms joined by a single bond are capable of free rotation about themselves. When two carbon atoms are joined by a double bond, they cannot rotate freely and the relative positions of the various groups attached to these carbon atoms get fixed. This gives rise to geometric isomers. Such compounds, which possess the same structural formula but differ in the spatial arrangement of the groups about a double bond are called geometrical isomers. The phenomenon is known as the geometrical isomerism or (also known) as cis - trans isomerism.

In cis-form, the like groups lie on the same side.

In trans-form, the like groups lie on the opposite sides.

If one of the sp2 carbons of the double bond is attached to two identical substituents, there is only one possible structure for the alkene. In other words, cis and trans isomers are not possinle for an alkene that has identical substituents attached to one of the double-bonded carbons.

Functional isomerism

Alkenes and cycloalkanes have the same molecular formula (CnH2n). Since cycloalkanes do not behave as unsaturated hydrocarbons, alkenes and cycloalkanes are termed functional isomers. For instance, propene is isomeric with cyclopropane (or trimethylene).

        CH3-CH=CH2  

               propene                

PHYSICAL PROPERTIES

Physical state

The first lower member like ethene, propene and butene are colorless gases. Alkenes with five to fifteen carbon atoms are liquids and higher ones are solids at ordinary temperatures. Alkenes have characteristic smell.

Density

Alkenes are lighter than water.

Solubility

Alkenes are insoluble in water and soluble in organic solvents such as benzene, ether etc.

Boiling point

The boiling points of alkenes gradually increase with an increase in the molecular mass (or chain length). This indicates that inter-molecular attractions become stronger with an increase in the size of the molecule. Branched chain alkenes have lower boiling points than the corresponding straight chain isomers. This is because cis-isomers are polar molecules and have higher boiling points when compared to the trans-isomers.

cis-2-butene (b.p.= 3.7°C) and trans-2-butene (b.p.= 1°C)

Melting point

The melting points of alkenes increase with an increase in the molecular mass. The trans-isomers fit well into the crystal lattice due to their symmetrical structure. This results in their melting at higher temperatures as compared to the cis form.

CHEMICAL PROPERTIES

Alkenes are more reactive than alkanes due to the presence of a double bond. The carbon-carbon double bond consists of a strong bond and a weak p bond. The typical reactions of alkenes involve the breaking of this weaker p bond, viz., and formation of two sigma (s) bonds.

Such reactions are called addition reactions.

Addition of halogen acids

In asymmetrical alkenes, the addition of a halogen acid takes place in a manner where by the halogen atom (the negative part of the molecule to be added) adds to the carbon atom, which has lesser number of hydrogen atoms on it.

This rule of the addition of halogen acids to an asymmetrical alkene is known as Markownikoff's rule (1869)

SUMMARY OF CHEMICAL REACTIONS OF ALKENES

     Example   How many grams of HCl solution which is 73% by mass are needed to saturate 16,8 liters  of propene at STP?

Solution:

Write the equation for chemical reaction

CH3CH=CH2 + HCl → CH3CH(Cl)-CH3

Find the mole number of propene n= 16,8/ 22.4 = 0,75 mole

According to chemical equation n(C3H6) = n(HCl) = 0,75 mole

Find the mass of HCl, m(HCl) = n*M = 0,75 * 36,5 = 27,375 g

Find the mass of HCl solution m= 27,375 *100 / 73 = 37,5 g

A test to distinguish between ALKANE and ALKENE hydrocarbons

Hydrocarbons are colourless. Bromine dissolved in water or trichloroethane solvent forms an orange (yellow/brown) solution.

When bromine solution is added to both an alkane or an alkene the result is quite different.

The alkane solution remains orange - no reaction.

However, the alkene decolourises the bromine as it forms a colourless dibromo-alkane compound - see equations.

     Example:  5,6 liters of H2 at STP is used to saturate a mixture of C2H4 and C3H6. When the products are burned 13,44 liters of CO2 is released at STP. What is the mass percentage of ethylene in the original mixture?

    Solution:

Write the chemical equations

        x       x         x                       x       3,5x        2x        3x

C2H4 + H2 → C2H6                  C2H6 + 7/2O2 → 2CO2 + 3H2O

   y       y        y                        y        5y        3y        4y

C3H6 + H2 → C3H8                   C3H8 + 5O2 → 3CO2 + 4H2O

Find the mole number of hydrogen n(H2) = 5,6 / 22,4 = 0,25 mole

Find the mole number of CO2 n(CO2) = 13,44/22,4 = 0,6 mole

Let’s take that in the mixture we have x mole of C2H4 and y mole of C3H6,

     then x+y = 0,25 (by first two equations), and

           2x+  3y = 0,6

By solving the system of equation representing x as 0,25-y we have

     2 (0,25 –y) + 3y = 0,6

0,5 – 2y + 3y = 0,6  

y = 0,1 mole of propene and x = 0,15 mole of ethylene

     6.   Find mass of C2H4 and C3H6

             m(C2H4) = 0,15 * 28 = 4,2 g

             m(C3H6) = 0,1 * 42 = 4,2 g

Find the mass of mixture m= 4,2 + 4,2 = 8,4 g

Find mass percentage of C2H4, w = 4,2 *100/ 8,4 = 50%

PREPARATION OF ALKENES

In the laboratory, alkenes are usually prepared from either alcohols or haloalkanes (alkyl halides).

1. By the dehydration of alcohols

Alkenes are obtained by the dehydration of alcohols. The dehydration of alcohols can be affected by two common methods.

- By passing the vapors of an alcohol over heated alumina.

- By heating an alcohol with concentrated mineral acid, such as concentrated H2SO4 or concentrated H3PO4· Anhydrous zinc chloride can also be used as a dehydrating agent.

By passing the vapors of an alcohol over alumina (Al2O3) at 623K (350°C).

The loss of water from an alcohol to give an alkene does not occur in just one step; a series of steps are involved in the mechanism of dehydration of alcohols. In the dehydration reaction given above, the following steps are involved.

First, the acid protonates (adding a proton or H+) the alcohol on the most electronegative atom, mainly oxygen. This process is usually reversible.

In the second step, the protonated alcohol loses water to give a positively charged species known as a carbonium ion or carbocation.

Finally the carbonium ion loses a proton to give alkene.

The mechanism of dehydration of ethyl alcohol is described below.

2. From haloalkanes (or, alkyl halides)

Alkenes can be obtained from haloalkanes (alkyl halides). These haloalkanes are usually bromo and iodo and less commonly, chloro derivatives.

By dehydrohalogenation of haloalkanes

Haloalkanes on heating with alcoholic potash loses one molecule of hydrogen halide to give alkene.

If two alkenes may be formed due to dehydrohalogenation of a haloalkane, the one which is most substituted is the main product. For example, dehydrohalogenation of 2-bromobutane gives,

The order of reactivity of haloalkanes in dehydrohalogenation is,

Tertiary > Secondary > Primary

Note: Reactions in which a small molecule like H2O or HX is lost are known as elimination reactions.

From vicinal dihaloalkanes

Vicinal dihaloalkanes are those dihalogen derivatives of alkanes in which two halogen atoms are on the adjacent carbon atoms. Alkenes can be obtained from vicinal dihaloalkanes by dehalogenation. When such a dihaloalkane is heated with zinc in methanol, an alkene is formed.

3. From alkynes

Alkenes can be easily obtained by hydrogenation of alkynes. An alkyne on controlled hydrogenation with hydrogen in the presence of Ni or Pd at 200°C give corresponding alkene.

                                  CnH2n-2 + H2       CnH2n

Ethyne gives ethene on hydrogenation.

    Example

      For 23 grams of ethyl alcohol

How many grams of ethylene can be obtained from a reaction having a 70% yield?

How many grams of 40% Br2 solution is needed to saturate obtained ethylene?

   Solution:

Write the equation for chemical reaction

                                H2SO4

              C2H5OH       →     C2H4 + H2O

Find the mole number of ethyl alcohol

n= m/ M = 23/ 46 = 0,5 mole

According to chemical equation n(ethanol)= n (ethylene) = 0,5 mole

Find the mass of ethylene m= 0,5 * 28 = 14 g

Since the yield of product is only 70% - m (ethylene) = 14 * 0,7 = 9,8 g

Write the reaction of ethylene with bromine

C2H4 + Br 2 → C2H4Br2

Find mole number of bromine, according to chemical equation n(Br2)= n(C2H4) = 0,5 mole

Find mass of bromine m= 0,5 * 160 = 80 g

Find mass of bromine solution m = 80*100/40 = 200 g

SOME COMMON POLYMERS

When catalysed and heated under pressure, unsaturated alkenes link together when 'half' of the double bond opens. The spare bonds are used to join up the molecules. The general equation for polyalkene formation is shown in the diagram above.

The original small molecule is called the monomer and the long molecule is called the polymer, which is the sort of molecule most plastics consist of. The polymer is now a saturated molecule but has the same C:H ratio as the original alkene.

Polymers are classified based on their source of availability as follows:

1. Naturally occurring polymers

2. Semi synthetic polymers

3. Synthetic polymers

Natural Polymers

These occur in nature in plants and animals and are very essential for life. For e.g., proteins constitute much of the animal body., nucleic acids control heredity at molecular level and cellulose provides food, clothing and shelter.

Examples: Starch, cellulose, protein, silk, wool and natural rubber are some natural polymers

Semi Synthetic Polymers

Semi synthetic polymers are derived from naturally occurring polymers by chemical modifications.

Examples: Vulcanized rubber, gun cotton and cellulose diacetate.

Vulcanized rubber is used in making tyres. Gun cotton which is cellulose nitrate is used in making explosives. Cellulose on acetylation with acetic anhydride in the presence of sulphuric acid forms cellulose diacetate used in making threads and materials like films, glasses etc.

Synthetic Polymers

Synthetic polymers are man-made polymers which include fibers like teflon and dacron, synthetic rubbers, plastics and PVC.

Synthetic polymers are grouped into two categories: addition polymers and condensation polymers.

Lots of small molecules join up to form a big long molecule in a process with no other molecules produced called addition polymerisation and the polymers are named as poly (name of original alkene), i.e. poly(alkene). Polyethylene is an example of an addition polymer.

A condensation polymerization occurs when a polymer is formed by a reaction that leaves behind a small molecule, such as water.

POLYETHYLENE

Poly(ethene) from ethene is a cheap but very useful plastic used for plastic bags and bottles (old or commercial names: polyethylene, polythene and polyethene).

It is obtained by polymerization of ethylene under a high pressure of 1000 to 2000 atmospheres at a temperature of 350 to 570 K in the presence of traces of oxygen or a peroxide which initiates the polymerization.

LDP (Low density polythene) is chemically inert, tough but flexible and a poor conductor of electricity.

Hence it is used in insulation of electric wires, manufacturing of squeeze bottles, toys, plastic bags and flexible pipes.

When polymerization of ethylene is carried out in the presence of a catalyst at about 330 to 350 K, at atmospheric pressure, the polymer obtained has a linear structure and is called HDP (High density polythene).

It is also chemically inert but relatively tough and hard with high tensile strength and is used in the manufacture of containers, house wares, bottles, pipes etc.

POLYVINYL CHLORIDE (PVC)

Poly(chloroethene), PVC, made from chloroethene (old name vinyl chloride) is much tougher than poly(ethene) and very hard wearing with good heat stability. So it is used for covering electrical wiring and plugs. It is also replacing metals for use as gas and water drain pipes and has found a use as artificial leather and readily dyed to bright colours! (old names : polyvinyl chloride, shortened to PVC)

 POLYPROPYLENE

Poly(propene) from propene is stronger and more hard wearing than polythene and is used for making crates, fibres and ropes (old or commercial names: polypropylene, polyprene and polypropene).

Many plastic items for medical or laboratory use can be made from polypropylene because it can withstand the heat in an autoclave. Its heat resistance also enables it to be used as the manufacturing material of consumer-grade kettles. Food containers made from it will not melt in the dishwasher, and do not melt during industrial hot filling processes.

 POLYSTYRENE

Polystyrene* is made from styrene* (another alkene monomer) and is used in making products such as toys, display boxes, packaging material, egg cartoons and styrofoam.

[*new names: poly(phenylethene) polymer made from phenylethene monomer]

POLYTETRAFLUOROETHYLENE (TEFLON)

Polytetrafluoroethylene, known as Teflon, is used in the production of nonstick cooking ware.

SUPPLEMENTARY QUESTIONS

1. Give the IUPAC name of the following compounds:

a.

b.  

         c.

         d.  

e.

f.

g.

2. Write the structural formulae for the following compounds.

1,2-dichloroethene

2,3-dimethyl-2-pentene

1,3-dibromocyclohexene

Cis-3,4-dibromo-3-hexene

1,3-pentadiene

2-bromo-3-methyl-3-hexene

3,5-divinylcyclopentene

2,3-dimethyl-2-butene

4-ethyl-3-methyl-3-octene

3-bromocyclopentene

2,3,4-trimethyl-4-bromo-2-hexene

Draw the structural formulas of isomers of C6H12 and name all compounds.

Find the molecular formula of the cycloalkene that contains 10% hydrogen by mass.

The density of a gaseous alkene is 2,5 g/l at STP. Find its molecular formula.

Write the equations for the reactions between propene and the given substances: hydrogen, bromine, hydrogen bromide and water. Name products.

At STP 4,48 liters of hydrogen is needed to saturate 1 mole of a mixture of ethane and propene. What is the mass of ethane in mixture?

How many grams of HCl solution which is 73% by mass are needed to saturate 16,8 liters of propene at STP?

Explain Markovnikov’s rule and complete the reactions below.

1-butene + HCl →

2-methyl-2-pentene + HBr → 

2-butene + HI →

Write the molecular formulae of given compounds: tetrafluoroethylene, styrene, acrylonitrile, vinyl chloride, isobutene, isoprene. Find information about usage of polymers of these compounds.

Explain polymerization. Write the equation which represents the polymerization of vinyl chloride.

Write out 4 reactions for preparation of ethylene

13. How many grams of ethylene can be  produced from 230 grams of ethyl alcohol by 80% production?

14. An 8 liter sample of hydrogen can add to a 20 liter mixture of ethane and ethylene. What is the percentage by volume of ethane in the mixture?

15. How many grams of KMnO4 which is 40% pure is required to oxidize ethylene obtained from 9,2 grams of ethyl alcohol?

MULTIPLE CHOICE QUESTIONS

1. 2-chlorobutane is formed by the reaction of

1-butene and chlorine

1-butene and hydrogen chloride

2-butene and chlorine

butane and chlorine

2.  What is the product of the reaction of 2-butene with bromine?

1-bromobutane

2-bromobutane

1,2-dibromobutane

2,3-dibromobutane

3. By which reaction alkenes can be differed from alkane?

Bromine water

Copper wire

Ethanol

Litmus paper

4. Which one of the following does not undergo polymerization reaction?

Isopropene

Ethylene

Propylene

Ethane

5. What is the molecular formula of an alkene of which 0,3 mole weighs 21 grams?

      A. C2H2          B. C3H6         C. C4H10

D. C5H10         E. C6H12

6. How many sigma bonds are there in an alkene whose molecular weight is 56 g/mol?

A. 10      B. 11      C. 12      D. 13    E. 14

7. All of the followings are isomers of each other, except:

        A.   cyclohexane

        B.   2-hexene

        C.   3-methyl-1-pentene

        D.   1,3- hexadiene

        E.    3,3- dimethyl-1-butene

8. In which one of the following compounds cis-trans isomerism observed?    

       A. CH2=CH2

       B. CH3CH=CH2

       C. CH2=CH-CH2-CH3

       D. CH3CH=CH-CH3

       E. CH3-CH2-CH2CH=CH2

9. Which one of the following compounds undergoes an addition reaction with H2 in the presence of Pt catalyst?

A. CH4      B. C2H6    C. C2H4Cl2     D. C3H6

10. All of the following hydrocarbons undergo addition reactions except:

A. CH4     B. C2H4     C. C3H6     D. C4H8

11. 30 liters of air is needed to burn 1 liter of an alkene. What is the molecular formula of this alkene?

A. C2H4     B. C3H6    C. C4H8    D. C5H10

12. How many liters of oxygen at STP are required to burn the propene gas that is obtained from 1,2grams of propyl alcohol?

A. 1,344     B. 1,680   C. 1,792   D. 2,016

CH3CH=CH-CH3

For 1 mole of this compound

It undergoes an addition reaction with 1 mole of H2.

When it burns 4 moles CO2 is produced

It undergoes an addition reaction with 81 grams of HBr

Which of the above statement(s) is/are correct?

A. I only      B. I and II      C. I and III

D. II and III     E. I,II and III

14. What is the IUPAC name of the following compound for which its structural formula is given below?

2-ethyl-5-iodo- 2-methyl- 3-pentene

4-ethyl-1-iodo- 4-methyl -2-heptene

4-ethyl- 7-iodo- 4-methyl-3-pentene

1-iodo- 4-methyl- 4-propyl- 2-hexene

15. Which one of the followings is the formula of  cis-1,4-dichloro-2-butene ?

16.

What is the IUPAC name of given compound above?

A.         2-bromo-2-pentene

B.        3-bromo-3-butene

C.   2-bromo-3-methyl-2-pentene

D.         3-bromo-2-ethylbutene

Choose correct answers.

1. Alkenes can be produced by:

A. dehydrogenation of alkanes

B. hydrogenation of benzene

C. dehydration of alcohols

D. hydration of aldehydes

E. dehydrohalogenation

2. Propene will react with each of the compounds in the series:

A. KMnO4, H2O, H2SO4

B. NaOH, HCl, K2Cr2O7

C. H2, Cl2, O2

D. HBr, H2O, H2

E. Na, HI, KMnO4

3. Which ones are correct for both methane and propene?

A. bromination reaction

B. sp-hybridization of carbon atoms in molecule

C. existence of pi-bonds

D. hydrogenation reactions

E. combustion in air

F. slightly solubility in water

4.