Courses
Courses for Kids
Free study material
Free LIVE classes
More
JEE 2022 | Class 12

JEE Important Chapter - Preparation of Alkanes

Get plus subscription and access unlimited live and recorded courses
Preparation of Alkanes

Preparation of Alkanes


Last updated date: 21st Sep 2023
Total views: 140.1k
Views today: 2.42k

Alkanes are the class of hydrocarbons that are referred to as the saturated hydrocarbons, that is, hydrocarbons having all carbon atoms bonded to other carbon atoms or hydrogen atoms with single (sigma) bonds only. As the alkanes possess weak Van Der Waals forces, the first four members, C1 to C4 are gasses, C5 to C17 are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odourless. They are prepared in laboratories and industries through various techniques. Some techniques for the preparation of alkanes are discussed in this article.

Important Topics of Preparation, Properties and Reactions of Alkanes

  • Alkyl halide

  • Saturated Hydrocarbon

  • Halogenation

  • Hydrogenation

  • Pyrolysis

  • Decarboxylation 

  • Wurtz Reaction

  • Substitution Reaction

  • Grignard Reagent


Important Definition of Preparation, Properties and Reactions of Alkanes

Saturated Hydrocarbon

Only single bonds between carbon and hydrogen exist in saturated hydrocarbons.

Unsaturated Hydrocarbon

Carbon-carbon multiple bonds (double bonds, triple bonds, or both) are found in unsaturated hydrocarbons.

Hydrogenation

In the presence of finely split catalysts such as platinum, palladium, or nickel, dihydrogen gas reacts with alkenes and alkynes to create alkanes. This is referred to as hydrogenation.

Substitution Reaction

A substitution reaction is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group.

Pyrolysis

Higher alkanes break down into lower alkanes, alkenes, and other compounds when heated to a higher temperature. Pyrolysis, or cracking, is a heat-induced breakdown event that results in smaller fragments.

Grignard Reagent

A Grignard reagent or Grignard compound has the general formula RMgX, where X is a halogen and R is an organic group, usually an alkyl or aryl. 


Physical Properties of Alkanes

  • Because of the covalent nature of C-C and C-H bonds and the small variation in electronegativity between carbon and hydrogen atoms, alkanes are nearly non-polar molecules.

  • Gases (C1 to C4), liquids (C5 to C17), and solids (having 18 carbon atoms or more) make up the first four members.

  • They have no colour and emit no odour.

  • With increasing molecular mass and surface area, the boiling point rises steadily.


Chemical Properties of Alkanes

Alkanes are generally non-reactive in nature due to their nonpolar nature. But, it gives some reactions. The chemical reactions of alkanes are given below:


1. Halogenation Reaction

CH4 + Cl2 → CH3Cl + HCl           (in presence of sunlight)

CH3Cl + Cl2 +→ CH2Cl2 + HCl    (in presence of sunlight)

CH2Cl2 + Cl2 → CHCl3 + HCl       (in presence of sunlight)

CHCl3 + HCl  → CCl4 + HCl         (in presence of sunlight)

  • It is found that the rate of reaction of alkanes with halogens (X) is: F2 > Cl2 > Br2 > I2

  • Rate of replacement of hydrogens (H) of alkanes (R) is: 3° > 2° > 1°.


2. Combustion

CH4 + 2O2 → CO2 + 2H2O + energy        (Complete combustion)

CH4 + 2O2 → C + 2H2O                             (incomplete combustion)


3. Controlled Oxidation

2CH4 + O2 + Cu → CH3OH           (at 523K and 100atm)

CH4 + O2 + Mo2O3 → HCHO + H2O

2C2H6 + 3O2 + (CH3COO)2Mn→ 2CH3COOH + 2H2O

Normally, alkanes resist oxidation, however, potassium permanganate can oxidise alkanes with a tertiary H atom to the equivalent alcohols.

(CH3)3C-H + KMnO4 → (CH3)3C-OH


4. Isomerisation: The reagent anhydrous AlCl3 / HCl is used to convert linear hydrocarbons into chain hydrocarbons with the same number of atoms. The chemical formula of hydrocarbon remains the same only it gets converted from linear isomer to chain isomer. For example, the reaction of n-hexane with reagent anhydrous AlCl3 / HCl is shown below in which two chain isomers of n-hexane are formed.

Isomerisation


5. Aromatization: The aromatic hydrocarbons can be formed with linear hydrocarbons such as n-hexane with the help of d-block element’s oxides such as Cr2O3, V2O5 or MO2O3  at high temperature (773 K) and high pressure (10-20 atm). The reaction for the formation of benzene from n-Hexane is shown below.

Aromatization


6. Pyrolysis: The name pyrolysis suggests that a reaction involves fire energy to break molecules. The linear hydrocarbons such as n-hexane at high temperature ( 773 K ) break into smaller hydrocarbons such as butane, propane, ethane, and methane. The chemical reaction of pyrolysis for n-hexane is shown below.

Pyrolysis


Preparation of Alkanes

There are several methods of preparation of alkanes. Some of the important methods are given below:

1. Hydrogenation: Hydrogen is added to unsaturated hydrocarbons to form saturated hydrocarbons.
$CH_{3}CH=CH_{2}+H_{2}\xrightarrow{\text{Pt/Pd/Ni}}CH_{3}-CH_{2}-CH_{3}$

$CH_{3}-C\equiv C-H+2H_{2}\xrightarrow{\text{pt/pd/Ni}}CH_{3}-CH_{2}-CH_{3}$


2. From Alkyl Halide: Hydrogen is added in place of halogen in this reaction.

R-Cl + H2 + Zn + H+ → R-H + HCl


3. Wurtz Reaction: Two hydrocarbons combine to form a hydrocarbon in the presence of sodium in dry ether.

2R-X + Na + dry ether → R-R + 2NaCl


4. Decarboxylation Reaction: Sodium salt of carbonic acid gets converted into hydrocarbon in the presence of sodium hydroxide and calcium oxide.

CH3COO-Na+ + NaOH + CaO → CH4 + Na2CO3


5. Kolbe’s Electrolytic Method: Sodium salt of carbonic acid gets converted into higher hydrocarbons with help of the electrolysis process.

CH3COO-Na+ + 2H2O → CH3-CH3 + 2CO2 + H2 + 2NaOH   (on electrolysis)


Solved Examples Based on Preparation, Properties and Reactions of Alkanes

1. Sodium salt of which acid will be needed for the preparation of propane? Write a chemical equation for the reaction.

Ans: Sodium acid salt on decarboxylation gives alkanes. Butanoic acid sodium salt will be needed for the preparation of propane by a decarboxylation reaction.

CH3CH2CH2COO-Na+ + NaOH + CaO → CH3CH2CH3 + Na2CO3

Key Point: Sodium acid salt forms alkane by decarboxylation. The alkane produced in this reaction will have one carbon less from the parent sodium acid salt. 


2. Write IUPAC names of the products obtained by addition reactions of HBr to hex-1-ene in the absence of peroxide.

Ans: The addition of HBr to the alkene produces haloalkane. The haloalkane form will have halogen on the more alkylated carbon in the absence of peroxide. So, 2-Bromohexane will be produced in the given reaction.

2-Bromohexane will be produced in the given reactio


Key Point: In the absence of peroxide formation of haloalkane depends upon the stability of carbocation. 


Solved Problems of Previous Years’ Question Based on Preparation, Properties and Reactions of Alkanes

1. The correct structure of 2,6-Dimethyl-dec-4-ene is :


The correct structure of 2,6-Dimethyl-dec-4-ene


Ans: The correct answer for this question is option 1. Select the longest chain first, then give the lowest number to the double bond.


Select the longest chain first, then give the lowest number to the double bond.

Trick: IUPAC naming includes the selection of the longest possible chain. The numbering should be done according to the lowest number rule. Also, give priority for the lowest number to the double bond, triple bond and functional group in the chain.


2. CH3CH2COO-Na+ + NaOH → CH3CH3 + Na2CO3

Consider the above reaction and identify the missing reagent/chemical.

a. B2H6

b. Red Phosphorus

c. CaO

d. DIBAL-H

Ans: The given reaction is a decarboxylation reaction. This reaction takes place in the presence of sodium hydroxide (NaOH) and lime (CaO). 

Trick: The combination of NaOH and CaO is used in the proper fusion of the mixture that aids in the removal of the carbon dioxide from the reaction mixture.


3. Match List-I with List-II.

List- I

List- II


Hell-VolhardZelinsky reaction

Hell-VolhardZelinsky reaction


Gattermann-Koch reaction

Gattermann-Koch reaction


(c) R-CH2-OH + R’-COOH + Con. H2SO4

Haloform reaction

(d) R-CH2-COOH + x2/red Phosp. + H2O

Esterification

Choose the correct answer from the options given below.

(1) (a)-(iv), (b)-(i), (c)-(ii), (d)-(iii)

(2) (a)-(iii), (b)-(ii), (c)-(i), (d)-(iv)

(3) (a)-(i), (b)-(iv), (c)-(iii), (d)-(ii)

(4) (a)-(ii), (b)-(iii), (c)-(iv), (d)-(i)

Ans: The correct answer for this question is the fourth option. 

(a)-(ii), (b)-(iii), (c)-(iv), (d)-(i)

Trick: Esterification involves a reaction between alcohol and acid, Hell–Volhard–Zelinsky reaction takes place in the presence of red phosphorus and halogen,  Gattermann-Koch reaction involves CO in the presence of HCl and lewis acid, and Haloform reaction takes place in the presence of Sodium oxide halogen salt.

 

Practice Questions

1. An alkene ‘A’ on ozonolysis gives a mixture of ethanal and pentan-3- one. Write structure and IUPAC name of ‘A’. 

(Ans: 3-Ethylpent2-ene)

2. Why is benzene extraordinarily stable though it contains three double Bonds? 

(Ans: due to resonance)


Conclusion

In this article, we have provided important information regarding the chapter preparation, properties and reactions of alkanes such as important concepts, properties, reactions, etc. Students should work on more solved examples and previous years’ question papers for securing good grades in the JEE Advanced exams. 

See More
JEE Advanced Important Dates

JEE Advanced Important Dates

View all JEE Advanced Exam Dates
JEE Advanced 2023 exam date have been announced by the NTA. JEE Advanced 2023 will now be conducted on 4-June-2023, and the exam registration closes on 7-May-2023. You can check the complete schedule on our site. Furthermore, you can check JEE Advanced 2023 dates for application, admit card, exam, answer key, result, counselling, etc along with other relevant information.
See More
View all JEE Advanced Exam Dates
JEE Advanced Information

JEE Advanced Information

Application Form
Eligibility Criteria
Reservation Policy
Admit Card
IIT Guwahati has announced the JEE Advanced 2023 application form release date on the official website https://jeeadv.ac.in/. JEE Advanced 2023 Application Form is available on the official website for online registration. Besides JEE Advanced 2023 application form release date, learn about the application process, steps to fill the form, how to submit, exam date sheet etc online. Check our website for more details.
JEE Advanced 2023 applicants should be aware of the eligibility criteria before applying to the exam. IIT Guwahati has released all the relevant information on the official website, i.e. https://jeeadv.ac.in/. JEE Advanced 2023 aspirants should have passed Class 12th or any other equivalent qualifying examination in 2022, 2021, or students appearing in the Class 12th final exam in 2023 can also apply. For further details, visit our website.
As per the union government’s norms, IIT Guwahati has released the JEE Advanced 2023 reservation criteria for different candidates’ categories (SC/ST/OBC/PwD), All India Quota, State Government Quota, Deemed Universities, and more. You can check more details on Vedantu website.
JEE Advanced 2023 Admit Card will be released by the IIT Guwahati in the month of August 2023. Candidates can download the JEE Advanced admit card and hall ticket from the JEE Advanced official website i.e. https://jeeadv.ac.in/. For more details on the JEE Advanced admit card 2023, candidates can check Vedantu official website.
It is crucial for the the engineering aspirants to know and download the JEE Advanced 2023 syllabus PDF for Maths, Physics and Chemistry. Check JEE Advanced 2023 syllabus here along with the best books and strategies to prepare for the entrance exam. Download the JEE Advanced 2023 syllabus consolidated as per the latest NTA guidelines from Vedantu for free.
See More
Download full syllabus
Download full syllabus
View JEE Advanced Syllabus in Detail
JEE Advanced 2023 Study Material

JEE Advanced 2023 Study Material

View all study material for JEE Advanced
JEE Advanced 2023 Study Materials: Strengthen your fundamentals with exhaustive JEE Advanced Study Materials. It covers the entire JEE Advanced syllabus, DPP, PYP with ample objective and subjective solved problems. Free download of JEE Advanced study material for Physics, Chemistry and Maths are available on our website so that students can gear up their preparation for JEE Advanced exam 2023 with Vedantu right on time.
See More
All
Chemistry
Maths
Physics
See All
JEE Advanced Question Papers

JEE Advanced Question Papers

see all
Download JEE Advanced Question Papers & ​Answer Keys of 2022, 2021, 2020, 2019, 2018 and 2017 PDFs. JEE Advanced Question Paper are provided language-wise along with their answer keys. We also offer JEE Advanced Sample Question Papers with Answer Keys for Physics, Chemistry and Maths solved by our expert teachers on Vedantu. Downloading the JEE Advanced Sample Question Papers with solutions will help the engineering aspirants to score high marks in the JEE Advanced examinations.
See More
JEE Advanced 2023 Book Solutions and PDF Download

JEE Advanced 2023 Book Solutions and PDF Download

View all JEE Advanced Important Books
In order to prepare for JEE Advanced 2023, candidates should know the list of important books i.e. RD Sharma Solutions, NCERT Solutions, HC Verma books and RS Aggarwal Solutions. They will find the high quality readymade solutions of these books on Vedantu. These books will help them in order to prepare well for the JEE Advanced 2023 exam so that they can grab the top rank in the all India entrance exam.
See More
Maths
NCERT Book for Class 12 Maths
Physics
NCERT Book for Class 12 Physics
Chemistry
NCERT Book for Class 12 Chemistry
Physics
H. C. Verma Solutions
Maths
R. D. Sharma Solutions
Maths
R.S. Aggarwal Solutions
See All
JEE Advanced Mock Tests

JEE Advanced Mock Tests

View all mock tests
JEE Advanced 2023 free online mock test series for exam preparation are available on the Vedantu website for free download. Practising these mock test papers of Physics, Chemistry and Maths prepared by expert teachers at Vedantu will help you to boost your confidence to face the JEE Advanced 2023 examination without any worries. The JEE Advanced test series for Physics, Chemistry and Maths that is based on the latest syllabus of JEE Advanced and also the Previous Year Question Papers.
See More
JEE Advanced 2023 Cut-Off

JEE Advanced 2023 Cut-Off

JEE Advanced Cut Off
IIT Bombay is responsible for the release of the JEE Advanced 2023 cut off score. The qualifying percentile score might remain the same for different categories. According to the latest trends, the expected cut off mark for JEE Advanced 2023 is 50% for general category candidates, 45% for physically challenged candidates, and 40% for candidates from reserved categories. For the general category, JEE Advanced qualifying marks for 2021 ranged from 17.50%, while for OBC/SC/ST categories, they ranged from 15.75% for OBC, 8.75% for SC and 8.75% for ST category.
See More
JEE Advanced 2023 Results

JEE Advanced 2023 Results

JEE Advanced 2023 Result - IIT Bombay has announced JEE Advanced result on their website. To download the Scorecard for JEE Advanced 2023, visit the official website of JEE Advanced i.e https://jeeadv.ac.in/.
See More
Rank List
Counselling
Cutoff
JEE Advanced 2022 State Wise Rank List - The respective state authorities started releasing the JEE Advanced 2022 state merit lists for admission to Engineering course. Check JEE Advanced 2022 state-wise rank list for the year 2022 here.
The IIT Bombay will conduct JEE Advanced 2022 counselling at https://jeeadv.ac.in/. There will be six rounds of counselling for admission under All India Quota (AIQ), IITs. A mop-up round of JEE Advanced counselling will be conducted excluding 15% AIQ seats, while the dates of JEE Advanced 2022 counselling for 85% state quota seats will be announced by the respective state authorities.
IIT Bombay is responsible for the release of the JEE Advanced 2022 cut off score. The qualifying percentile score might remain the same for different categories. According to the latest trends, the expected cut off mark for JEE Advanced 2022 is 50% for general category candidates, 45% for physically challenged candidates, and 40% for candidates from reserved categories. For the general category, JEE Advanced qualifying marks for 2021 ranged from 17.50%, while for OBC/SC/ST categories, they ranged from 15.75% for OBC, 8.75% for SC and 8.75% for ST category.
Want to know which Engineering colleges in India accept the JEE Advanced 2023 scores for admission to Engineering? Find the list of Engineering colleges accepting JEE Advanced scores in India, compiled by Vedantu. There are 1622 Colleges that are accepting JEE Advanced. Also find more details on Fees, Ranking, Admission, and Placement.
See More
question-image

FAQs on JEE Important Chapter - Preparation of Alkanes

FAQ

1. How significant is the Preparation, Properties, and Reactions of Alkanes chapter in the JEE-advance exam?

In the exam, around 1-2 questions from this chapter can be expected, carrying approximately 8 marks or about 2% of the total marks.

2. Does acid react with alkane?

Nonpolar alkane molecules do not react with ionic chemicals like most laboratory acids, bases, oxidising agents, or reducing agents because they are nonpolar.

3. Do alkanes have no reactivity?

Most reagents do not react with alkanes for two reasons. First, due to excellent orbital overlap, carbon carbon and carbon hydrogen single bonds are extremely strong. Second, because the electronegativity of both elements is fairly comparable, the carbon hydrogen bonds make alkane molecules neither acidic nor basic.