• Engineering and Architecture
  • Management and Business Administration
  • Medicine and Allied Sciences
  • Law
  • Animation and Design
  • Media, Mass Communication and Journalism
  • Finance & Accounts
  • Computer Application and IT
  • Pharmacy
  • Hospitality and Tourism
  • Competition
  • School
  • Study Abroad
  • Arts, Commerce & Sciences
  • Learn
  • Online Courses and Certifications
Careers360 Logo
Careers360 Logo
Search Colleges, Exams, Schools & more
Exam Prep

NEET Preparation Of Aldehydes Concepts, Questions And Solutions

Subscribe to Careers360 Premium

Get Information and Premium Advisory Content - Exams, Colleges, Careers, Placements, Scholarships, Studying abroad etc. Engagement with Experts, Toppers, Subject matter faculty, Career Counselors, University and industry leaders.

Download and read later
By Sumit Saini
26 Jun'22  7 min read
NEET Preparation Of Aldehydes Concepts, Questions And Solutions
Synopsis

Preparation of Aldehydes is one of the most important topics for the NEET examination. There are various methods of preparation of Aldehydes which are discussed in detail below. Previous year questions are also discussed so that students get an idea of the difficulty level of the questions asked. 

Read more
NEET Preparation Of Aldehydes Concepts, Questions And Solutions
Synopsis

Preparation of Aldehydes is one of the most important topics for the NEET examination. There are various methods of preparation of Aldehydes which are discussed in detail below. Previous year questions are also discussed so that students get an idea of the difficulty level of the questions asked. 

Chemistry is one of the most scoring subjects in the National Eligibility cum Entrance Test (NEET), having a weightage of 25 percent out of three subjects – Chemistry, Physics and Biology which, in turn, includes Zoology and Botany. Considering the high NEET cut-offs in recent years, it is inevitable to prepare Chemistry in a comprehensive manner along with special focus on important topics.

Analysis :

In the last five years of NEET examination, there have been three direct questions from the topic “Preparation of Aldehydes” and many indirect questions. Some of the questions are discussed in detail towards the end of this article. The Questions asked on this topic are moderate to difficult. From the questions it can be concluded that one must memorise the reactions in order to solve the questions. Questions from this topic are based on the reaction mechanism in which reagent, intermediate or final product is asked. The topic “Preparation of Aldehydes” mainly belongs to the chapter Aldehydes, ketones and Carboxylic acids. However some of the portion of this topic is also discussed in the chapter “Alcohols, Phenols and Ethers”.

Preparation of Aldehydes

In organic Chemistry, the compounds having carbon-oxygen double bonds are known as Carbonyl compounds. Carbonyl group in Organic Chemistry is one of the most prevalent functional groups. Carbonyl functional group is predominantly found in Aldehydes and Ketones. Aldehydes are the compounds in which the Carbonyl group is attached to carbon and hydrogen whereas Ketones are compounds in which two Carbon atoms are attached to the Carbonyl group. In this article, we will be discussing mainly Aldehydes and their method of Preparation. Preparation of Aldehydes is an important topic for competitive examinations like JEE and NEET. Let’s discuss various methods of preparation of Aldehydes.

Preparation of Aldehydes can be undertaken through various methods depending upon the type of compound required. However, from the exam perspective and NCERT syllabus, we have some of the important reactions which are discussed below.

1. By Oxidation Of Alcohol

Oxidation of Alcohol is one of the prominent methods to prepare Aldehydes. However this can be achieved by Oxidation of primary alcohols only as in case of secondary alcohols, Ketones are formed. Below is the depiction of the preparation of Aldehydes via oxidation of Primary alcohols. Also preparation of Ketones is shown using secondary alcohol to understand the difference in oxidation product between primary and secondary alcohols.

The oxidation process in alcohol consists of the formation of a carbon-oxygen double bond and the cleavage of an O-H and C-H bonds as shown in the figure below.

1655972469164

The above mentioned processes of cleavage and formation of bonds occur in oxidation reactions. As it is clear from the above reaction, loss of dihydrogen takes place from alcohol molecules, hence this reaction is also known as dehydrogenation reaction.

For the Preparation of Aldehydes, oxidising agent used is CrO3 in anhydrous medium.

1655972500670

Another reagent which can be used for the oxidation of primary alcohols to Aldehydes is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl which is shown below.

1655972474719

Secondary alcohols are oxidized to ketone by chromic anhydride (CrO3 ) as shown below,

1655972474870

Apart from the primary and secondary alcohol oxidation, which are discussed above, Tertiary alcohols generally do not undergo oxidation reaction under normal conditions.

2. Dehydrogenation Of Alcohols

Dehydrogenation method to prepare alcohol is suitable for industrial application. In this method alcohol vapours are passed over metal catalysts like Ag or Cu such that the product obtained is an aldehyde. In this case also, Primary and secondary alcohols give Aldehydes and ketones, respectively. Below mentioned is the reaction depicting Dehydrogenation of Alcohols in which the vapours of a primary or a secondary alcohol are passed over heated copper at 573 K, dehydrogenation takes place and an aldehyde or a ketone is formed.

1655972490707

3. From Hydrocarbons

Formation of Aldehydes is undertaken by the ozonolysis of alkenes followed by reacting the ozonolysis products with zinc dust and water. In ozonolysis method, addition of ozone molecules to alkene takes place leading to formation of ozonide, after that cleavage of the ozonide by Zn-H2O to smaller molecules takes place. In ozonolysis reaction, a mixture of Aldehydes and ketones is obtained depending upon the structure of the hydrocarbon. Below mentioned are the reactions, in which ozonolysis product is shown which includes both Aldehydes and ketones depending on the reactant taken.


1655972501245

4. From Acyl Chloride

In this reaction, Acid chloride is passed over a catalyst like palladium on barium sulfate to obtain Aldehyde. This reaction is called Rosenmund reduction. Below is the representation of the reduction of acyl chloride in order to obtain aldehyde.

1655972479679

5. From Nitriles And Esters:

For the formation of Aldehydes, Nitriles are reduced to corresponding imine along with stannous chloride in the presence of hydrochloric acid, which on further hydrolysis give corresponding aldehyde. This reaction is known as the Stephen reaction. Below is the reaction representation of Stephen's reaction.

1655972475155

Similarly, to produce Aldehydes, esters are also reduced to Aldehydes with DIBAL-H which is shown below in the reaction.

1655972483851

6. From Hydrocarbons

This method is used to prepare aromatic Aldehydes from aromatic hydrocarbons.

Below are some of the methods to prepare aromatic Aldehydes.

a. By using chromyl chloride(CrO2Cl2):

In the process of formation of Benzaldehyde, Chromyl chloride (CrO2Cl2 ) is used in which Chromyl chloride oxidizes methyl groups to a chromium complex as shown in the reaction below, which on hydrolysis gives corresponding benzaldehyde.

This reaction is known as the Etard reaction.

1655972483706

b. By side chain chlorination followed by hydrolysis

This method is also known as the commercial method to manufacture benzaldehyde in which toluene is treated with chlorine in presence of UV light which leads to Side chain chlorination of toluene giving benzal chloride as shown in the reaction below. On further hydrolysis of benzal chloride, it gives benzaldehyde. Below is the reaction showing step-by-step formation of Benzaldehyde.

1655972483535

c. By Gatterman –Koch reaction

For the preparation of aromatic aldehyde, an important reaction is Gatterman-Koch reaction in which benzene or benzene derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminum chloride or cuprous chloride which gives benzaldehyde or substituted benzaldehyde. Below is the Gatterman-Koch reaction.

1655972475448

This reaction is Called Gatterman-Koch reaction.

Above discussed are some of the most important methods of preparation of Aldehydes. Students are advised to understand the above reactions in a logical manner and try to memorize the reactants, products and the reagents used in order to identify the reaction in the examination.

Below are some of the questions asked in NEET in the last 5 years from the topic “Preparation of Aldehydes”. Students should practice these questions by themselves after completing the theory in order to assess the difficulty level and type of questions asked in the NEET exam from this topic.

Q1. The Intermediate compound 'X' in the following chemical reaction is : [NEET 2021]

1655972486196

  1. 1655972482176


  1. 1655972484125

  2. 1655972483989

  3. 1655972484252


Solution. The reaction will be -

1655972507314

Therefore, option 1 is correct.

Q2. Identify the major products P, Q and R in the following sequence of reactions : [NEET 2018]

1655972449528

  1. 1655972479002

  2. 1655972448450

  3. 1655972448868

  4. 1655972448644

Solution. The reaction will be -

Due to Cl being a good leaving group, it will leave the alkane in the presence of anhydrous AlCl3 and forms 1-degree carbonation then convert to 2-degree carbocation.

1655972494857

Hence option 4 is correct.

Q3. Compound A, C8H10O, is found to react with

NaOI (produced by reacting Y with NaOH and yields a yellow precipitate with characteristic smell.

A and Y are respectively [NEET 2018]

  1. 1655972464384

  2. 1655972486489

  3. 1655972486372

  4. 1655972484692

Solution.

As we learnt in

Haloform Reaction -

CH_{3}COR\:\:is\:oxidised\:by\:\:NaOI\:\:to\:form\:iodoform\:and\:\:RCO_{2}Na

- wherein

C_{2}H_{5}COCH_{3}+NaOH+I_{2}\rightarrow C_{2}H_{5}CO_{2}Na+CHI_{3}

1655972491184

Hence Option 1 is correct

Q4. Consider the reactions: [NEET 2017]

1655972508578

Identify A, X, Y and Z.

  1. A-Methoxymethane, X-Ethanoic acid, Y-Acetate ion, Z-hydrazine.

  2. A-Methoxymethane, X-Ethanol, Y-Ethanoic acid, Z-Semicarbazide.

  3. A-Ethanal, X-Ethanol, Y-But-2-enal, Z-Semicarbazone.

  4. A-Ethanol, X-Acetaldehyde, Y-Butanone, Z-Hydrazone.

Solution. As we learnt in

Addition of ketones with alcohols -

Produces an unstable hemiketal intermediate and further stable compound called ketal.

- wherein

1655972508431

and

Addition of ammonia and its derivatives with aldehydes and ketones -

Product formation is favoured due to rapid dehydration of the intermediate to form > C = NG

G\:\:may\:be\:\:-OH,\:\:-NH_{2},\:\:-NHC_{6}H_{5},\:\:-NHCONH_{2},\:\:etc

- wherein1655972510778

and

Aldol condensation -

Aldehydes and ketones possessing \alpha - hydrogen atom in the presence of dilute alkali undergo self-condensation to form \beta - hydroxy aldehyde and

\beta - hydroxy ketone.

- wherein1655972511123

1655972510981

Hence option 3 is correct.

In the above article, we have discussed in detail the importance of the topic “Preparation of Aldehydes” for the NEET examination. This article can be referred by the students for the last minute revision as it includes all the important reactions which are related to Preparation of Aldehydes along with the question asked in the Previous years.

  • Education
  • Health care education
  • Medical education
  • Evaluation methods
  • Data collection
  • Student assessment and evaluation
  • Educational evaluation methods
  • Tests
  • Standardized tests

Subscribe to Premium Membership Plan

*Unlock all premium content and benefits:

  • Career Guidance

    Plan ahead with clear directions on courses, careers and best ways to learn

  • Fees & Scholarships

    Our guides on accessing loans, scholarships will help you understand how to finance your studies.

  • Jobs & Internships

    Prepare for employed life with tips on jobs, find paid & unpaid internship listings, summer jobs and career advice.

  • Interactive Counselling Webinars

    Webinars with academics and institutional leaders on admission, careers.

  • College Placement Reports

    Wondering what comes after college? Read placement reports of top universities, colleges.

  • Sports and Extra Curricular Activities

    Interested in music? Art? Summer camps? Start exploring here.

  • Mental Health & Well Being

    To help you deal with stress, face exams, stay focussed, experts share tips here.

  • Career Foundation

    It's not too early to train for work life. Read about building CVs, cracking job interviews; find paid & unpaid internship listings, summer jobs and career advice.

  • Study Help

    Understand concepts of Physics, Chemistry Maths and others through fun stories and illustrations

  • NCERT, Exemplar, CBSE Previous year Solutions for Maths & Science

    - Concept-oriented detailed solutions for NCERT (Class 6 to 12), Exemplar (Class 9 to 12) & CBSE Previous 5 year (Class 10 and 12)

    - For Maths and Science

  • 15+ B.Tech College Predictors

    Analyses your exam performace to predict admission chances based on Domicile, Caste, Gender, etc. and Exam Result.

    Get your personalised report with Top Predicted Colleges.

  • NCHMJEE Hospitality College Predictor

    - Know your college admission chances based on your Seat Type, Caste, etc. & exam result

    - Get your NCHM JEE personalised report with Top Predicted IHMs, Government & Private Colleges delivered via email

  • NIFT Design College Predictor

    - Know your admission chances based on your Home State, Caste, etc. & exam result for NIFT campuses

    - Get your NIFT personalised report for B.Des & B.FTech delivered via email

  • CLAT, AILET, DUET L.L.B, MHCET Law College Predictors

    - Know your admission chances in NLUs, Delhi University’s Faculty of Law for L.L.B (3Yr), National Law University, Delhi’s B.A-L.L.B course & Law colleges accross Maharashtra based on your Seat Type, Caste, etc. & exam results

    - Get your personalised report delivered via email

  • Study Abroad Guidance Webinars

    Interactive webinars with experts for different countries will take you through all the basics.

  • SOP Guidance

    What should go into your statement of purpose (SOP)? Learn how to put together a winning application.

  • English Proficiency Test

    How can you ace the English proficiency test? Let the experts help you.

  • Visa Regulations

    Have questions about visa regulations? Find answers here.

  • Career Support

    Exams and scholarship alerts, test prep reviews, study plans, college selection

Rate this Story

  • Poor
  • Weak
  • Good
  • Very Good
  • Excellent
Preparing For The SAT: Resources, Best Practices, And More
Read more from Latest Stories

Careers360 helping shape your Career for a better tomorrow

student

250M+

Students

colleges

30,000+

Colleges

exams

500+

Exams

ebook

1500+

E-Books