Preliminary Idea of Reaction Mechanism

 13.7       Preliminary Idea of Reaction Mechanism

Organic reactions are generally more complex and occur in multiple steps. So a detailed study of each step is required to understand a chemical reaction. The reaction mechanism is a detailed study of each stage of a reaction with special attention on the reactants, products, intermediates, energy changes involved, and rates of each step.

Some fundamental knowledge is necessary to study the reaction mechanisms, which are discussed here.

Concept of Bond Breaking/ Cleavage/ Fission

During a chemical reaction, existing chemical bonds break and new bonds form. Organic compounds have covalent bonds. Hence organic reactions involve the breaking of existing and the formation of new covalent bonds. A covalent bond undergoes cleavage in either of the following two ways.

1.      Homolysis (Homolytic bond fission/ Symmetrical or non-polar bond cleavage)

It is the breaking of bonds with an equal distribution of bonding electrons among bonding atoms. It generally takes place when bonding elements have nearly equal electronegativities. Typically it takes place in the gas phase in the presence of heat or sunlight. e.g.,

 

This leads to the formation of free radicals.

2.      Heterolysis (Heterolytic bond fission/Unsymmetrical or polar bond fission)

It is the breaking of bonds where one of the bonding atoms takes both the electrons and the other gets none. It generally takes place when bonding elements have a large difference in electronegativities. It generally takes place in polar solvents.

Heterolysis leads to the formation of cations and anions.

Types of Reagent

Generally, reactants of organic reaction involve two species: the main organic molecule called substrate and a species that attacks the substrate, called reagent or attacking species.

Nucleophile (Greek: Nucleo = Nucleus; phile = loving)

Atom or groups of atoms that is electron-rich species that attack electron-deficient centres.

Nucleophiles can be negatively charged ions like OH^-, CN^-, I^-, OR^- or neutral molecules with lone pairs of electrons like H₂O, NH₃, ROH, ROR, RNH₂ etc.

Electrophile (Greek: Electro = Electron; phile = loving)

Atoms or groups of atoms that are electron-deficient species that attack electron-rich centres.

Electrophiles may be positively charged ions like H⁺, Cl⁺, Br⁺_, NO₂⁺, R⁺ etc. or neutral electron deficient species like BF₃, AlCl₃, FeCl₃, CO₂, SO₃ etc.

1.       Define electrophile and nucleophile with one example each.              2

2.       How is an electrophile different from a nucleophile? Give a suitable example of both.     2

3.       Distinguish between electrophile and nucleophile with an example of each.

4.       How is electrophile different from nucleophile? Give an example of each.

5.       What is meant by electrophile? Write suitable examples of it.           2

6.       Explain nucleophiles giving examples.                                               2

Reaction Intermediates

Free radicals

A free radical is an atom or group of atoms with an odd or unpaired electron. These are short-lived and very reactive reaction intermediates with no net charge. The extreme reactivity of the free radicals is due to the tendency of the unpaired electron to get paired. These are a result of homolysis. e.g.

The relative stabilities of different degrees of alkyl free radicals are:

Electronic Transitions in the covalent bonds (Organic Effects)

Inductive effect (I effect)

When two atoms having different electronegativities are linked together by a covalent bond, the electron pair shifts towards the more electronegative atom resulting in a certain degree of polarity in the bond. When a carbon atom is bonded to a hydrogen atom or another carbon atom by a covalent bond as in alkane, the sharing of electron pairs is symmetrical between them, and the bond is non-polar.

Thus, the polarity produced in a molecule due to the higher electronegativity of one atom compared to another is called an inductive effect. This type of polarity migrates in the carbon chain. It is indicated as arrows in the direction of electron displacement. This type of polarity migrates in the carbon chain. It is relayed up to the third or fourth carbon atom and fades afterwards.

            C         C         C         Cl^δ-

The following two inductive effects are observed depending on the nature of groups attached to a C chain.

[A] Positive Inductive effect (+I effect):

If the group attached to C- atom pulls the bonded electrons to a lesser extent than an H atom, the C atom gets a partial negative charge; the group gets a partial positive charge. Such an inductive effect is called a positive inductive effect. Such groups are called electron-releasing groups. E.g., -CH₃, -COO^-, -CH₂R, -CHR₂, -CR₃ etc.

The + I effect of the different alkyl groups is in the following order.

[B] Negative Inductive effect (-I effect)

If a group attached to the carbon atom pulls the electrons to a more extent than an H – atom, the bond pair of electrons are displaced towards the group. Such groups get a partial negative charge; the carbon atom gets a partial positive charge. Such an inductive effect is called a negative inductive effect. Such groups are called electron-withdrawing groups. e.g., -NO₂, -CN, -COOH etc.

These groups are arranged in the increasing order of their –I effect as:

-NO₂ > -CN > -COOH > -F > -Cl > -Br > -I etc.

                                Resonance effect (R-effect)

The phenomenon where a molecule or ion couldn’t be represented by a single structure but rather should be represented by more than one structure to justify its properties is called resonance.

Such structures are called resonance structures and the actual structure is considered a resonance hybrid.

The increase or decrease in electron density in some parts of a molecule due to the existence of resonance is called the resonance effect. The resonance effect can also be positive or negative.

-R effect of aryl group (phenyl group) in Chlorobenzene:

-R effect of aryl group (phenyl group) in Phenol:

+ R effect of aryl (phenyl) group in nitrobenzene

7.       What is meant by inductive effect? Give its one application.             2

8.       Write a short note on the inductive effect.                                         5

1.       Homolytic bond fission results in the formation of

a.       Carbocations                                 c. Carbanion

b.       Free radicals                                 d. Nitrenes

2.       Homolytic fission of carbon to carbon covalent bond gives

a.       Carbocations                                 c. Carbanions

b.       Ion radicals                                    d. Free radicals

3.       Heterolytic fission of carbon to carbon bond gives

a.       Free radicals                                 c. Charged species

b.       Neutral species                            c. None of these

4.       Which of the following statements is true regarding nucleophiles?

a.       They have an unpaired electron.

b.       They have a lone pair of electrons.

c.        They have empty orbitals.

d.       They have an overall positive charge.

5.       Which of the following is nucleophile?

a.       AlCl₃                       b. NH₃                               c. Br⁺                                  d. NO₂⁺

6.       Which of the following is not a nucleophile?

a.       OH^-                          b. NH₂^-                              c. BF₃                                 d. NH₃

7.       Which of the following can act as a nucleophile?

a.       BF₃                           b. FeCl₃                            c. ZnCl₂                            d. C₂H₅MgBr

8.       Which of the following is electrophile?

a.       NH₃                          b. H₂O                    c. CN^-                                 d. SO₃

9.       Which of the following is an electrophile?

a.       Cl^-                              b. H₂O                    c. BF₃                                 d. ROH

10.    Which of the following is not an electrophile?

a.       H₃O⁺                       b. CN^-                                 c. AlCl₃                             d. Br^-

11.    Which of the following statements is not true?

a.       Free radicals are electron deficient.

b.       Free radicals are the results of homolytic fission.

c.        Free radicals carry a positive charge.

d.       Free radicals have sp² hybridization.

12.    The presence of chlorine in an organic compound causes

a.       -I effect          b. + I effect            c. Resonance        d. None of them

13.    Which of the following groups exert the +I effect?

a.       -CH₃                        b. –Cl                      c. –COOH               d. -NO₂

14.    Transfer of electrons in multiple bonds in the presence of an attacking reagent is

a.       Electromeric effect                     c. Hyperconjugation

b.       Inductive effect                            d. Resonance

* This note is insufficient. It is to guide you. Study the prescribed textbooks too. *

 

Fundamental Principles of Organic Chemistry

13.3 Isomerism in Organic Compounds

Isomers are different compounds possessing the same molecular formula. Isomerism occurs when the same chemical formula represents distinct substances.

Types of Isomerism:

Isomerism is of two types:

1.       Structural isomerism

2.       Stereoisomerism   

1.       Define isomerism.                                                                               1

13.5 Structural isomerism and its types

Structural isomerism

Structural isomerism is the term used to describe molecules with the same chemical formula but distinct structures. The 5 types of structural isomers are as follows:

 

1.      Chain, nuclear or skeletal isomerism

If two or more compounds have the same molecular formula but different carbon chains or skeletons, they are called chain isomers; the phenomenon is called chain isomerism.

For example, the two isomers of C₄H₁₀ differ only in the length of the carbon chain.

Similarly, butyl alcohol and isobutyl alcohol are chain isomers.

2.      Position/positional isomerism

The compounds with the same molecular formula but different positions of the functional group are termed position isomers; the phenomenon is position isomerism.

For example, the two isomers with molecular formula C₃H₈O are:

3.      Functional isomerism

The compounds with the same molecular formula but different functional groups are termed functional isomers; the phenomenon is functional isomerism.

For example

1.       Aldehydes and ketones, e.g. CH₃CH₂CHO and CH₃COCH₃

2.       Alcohol and ether, e.g. CH₃CH₂OH and CH₃OCH₃

3.       Carboxylic acid and esters, e.g.

4.      Tautomerism

It is a special isomerism in which the isomers are interconvertible due to the change in the position of the hydrogen atom within the molecule. For example:

5.      Metamerism

The compounds of the same family with the same molecular formula, which differ in the arrangement of carbon atoms on either side of the functional group, are metamers; the phenomenon is metamerism. Ketones, Ethers, Esters etc. can exhibit metamerism.

2.       Write a short note on structural isomerism in organic compounds.  5/2

3.       Write the isomers of C₄H₁₀ and give their IUPAC names.                  2

4.       Write down two structure isomers and their IUPAC names of C₄H₈. 2

5.       Write short notes on isomers of C₆H₁₀.                                               5

6.       Give a functional isomer of C₃H₇OH and write its IUPAC name.2

7.       Write down the functional isomers of CH₃-CH₂-CH₂-OH and give IUPAC name.        2

8.       Write down the functional isomer of Propanol and give its IUPAC name. 1

9.       Write down the functional isomers of CH₃-CH₂-OH and give IUPAC name.   2

10.   Give a functional isomer of C₂H₄O₂ and write its IUPAC name.         2

11.   Give the functional isomer of Ethanoic acid with structural formula.       1

12.   Write down the functional isomer of Propanoic acid and give its IUPAC name.          2

13.   Write down the functional isomer of Methyl methanoate and give its IUPAC name.  1

14.   Write down the functional isomers of CH₃-CH₂-COOH and give IUPAC name.         1

15.   Give possible isomers of C₃H₆O₂ and write their IUPAC names.        2

16.    Write down two structure isomers and their IUPAC names of C₃H₆O.    2

17.   Give the functional isomer of Propanone with structural formula.            1

18.   Write the isomers of C₂H₆O and give their IUPAC names.                      2

19.   Write down the functional isomer of Methoxymethane and give its IUPAC name.      1

20.   Give possible isomers of C₃H₆O and write their IUPAC names.              2

Write possible isomeric ethers for the molecular formula C₄H₁₀O.

13.6 Concept of geometrical isomerism (cis & trans) & optical isomerism (d & l form)

Compounds with the same molecular formula and the structural formula but different spatial arrangements of the atoms or groups of atoms in a molecule are called stereoisomers. The phenomenon of the existence of such compounds is called stereoisomerism.

Stereoisomerism is subdivided into geometrical and optical isomerism.

Geometrical Isomerism (cis, trans isomerism)

When the same molecular formula represents two compounds which differ in the spatial arrangement of atoms or groups around the carbon-carbon double bond, these are geometrical isomers. Such isomerism arises due to the inability to rotate around a double bond.

e.g., But-2-ene has two geometrical isomerism i.e., cis-but-2-ene and trans-but-2-ene

1,2-dichloroethene CHCl=CHCl also exhibits such isomerism.

            Optical isomerism

·         Optical isomerism is the phenomenon in which isomers have different arrangements of atoms or groups in three-dimensional space.

·         Such compounds resemble in their chemical and most physical properties but differ in optical rotation.

·         Optical rotation is a phenomenon where optically active compounds rotate the plane of plane-polarized light in a clockwise or anticlockwise direction.

·         Polarimeter measures the optical rotation.

·         The optical isomers that are mirror images of each other are called enantiomers.

·         The enantiomers which rotate the plane of the polarized light to a clockwise direction (right) and anticlockwise direction are called dextrorotatory and laevorotatory respectively, designated as + or d and - or l respectively.

Only compounds having a chiral carbon atom exhibit enantiomerism. A carbon atom bonded with 4 different groups is called chiral carbon. In the above four examples, the central atom is chiral.

 

1.       Compounds with the same molecular formula but possessing different properties are

a.       Isotopes         e. isomers              c. isobars               d. isochors

b.       isotherms

2.       Compounds having the same molecular formula but different structural formulas are

a.       Isotopes                         c. Structural isomers

b.       Isobars                           d. polymers

3.       Isomers must have

a.       Same chemical properties         c. Same structural formula

b.       Same molecular formula            d. Same functional group

4.       Structural isomers have the same molecular formula but different

a.       Structure                                       c. Functional group

b.       Physical properties                      d. Chemical properties

5.       Two isomeric forms of saturated hydrocarbons have

a.       Same structure

b.       Same molecular formula

c.        Same properties

d.       Different compositions of elements

6.       CH₃-O-CH₃ and C₂H₅OH are

a.       Isomers          b. Allotropes         c. Isobars               d. Isomorphs

7.       Which of the following compound is a functional isomer of C₂H₅OH?

a.       CH₃CHO         b. CH₃COOH          c. (CH₃)₂O              d. (C₂H₅)₂O

8.       How many structural isomers does pentane have?

a.       4                      b. 5                         c. 6                          d. 7

9.       Chain isomers differ in

a.       Molecular formula                       c. Empirical formula

b.       Molecular weight                         d. Length of the carbon chain

10.    Which compound is not an isomer of the other three?

a.       Pentane                                         c. 2-methylbutane

b.       2,2-dimethylpropane                  d. 2,3-dimethylbutane

11.    But-1-yne and But-2-yne are

a.       Chain isomers                               c. Metamers

b.       Functional isomers                      d. Positional isomers

12.    Metamerism is shown by

a.       Ketones         b. Alkanes              c. Aldehydes         d. Alcohols

13.    The number of possible alcoholic isomers of C₄H₁₀O is

a.       2                      b. 3                         c. 4                          d. 8

14.    How many isomeric ethers are possible from C₄H₁₀O?

a.       2                      b. 3                         c. 4                          d. 5

15.    The maximum  number of isomeric alkenes with molecular formula C₄H₈ is

a.       2                      b. 3                         c. 4                          d. 5

16.    Which of the following are isomers?

a.       Propanone and Ethanal

b.       Ethanol and Methoxymethane

c.        Propanone and Propanoic acid

d.       Methanol and Methoxymethane

17.    Isomerism where a single compound exists in two or more readily interconvertible structures is

a.       Chain isomerism                          c. Metamerism

b.       Tautomerism                                 d. Stereoisomerism

18.    …………….. generally exhibit geometrical isomerism.

a.       Alkanes          b. alkenes              c. alcohols             d. alkynes

19.    cis-but-2-ene and trans-but-2-ene are

a.       position isomers                          c. optical isomers

b.       conformational isomers             d. geometrical isomers

20.    Optical activity is measured by

a.       Nicol prism    b. Analyzer            c. Polarimeter       d. Polaroid

21.    Which of the following compound is chiral?

a.       2-chloropropane         c. 2-methoxybutane

b.       Propan-2-ol                   d. Methanal

22.    But-1-ene exhibits

a.       Geometrical isomerism              c. optical isomerism

b.       Position isomerism                      d. none of these

 

***This is not a complete note. It is to guide you. Study the prescribed textbooks along with this material. ***