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MDCAT/ECAT Chemistry Hydrocarbons Quiz #5 🧬 | Hydrocarbons High-Yield Chemistry MCQs💥 Exam-Focused, 🎯 💯 Exam Booster🚀 Practice & Win 190+! 📚 Topper’s Choice

1️⃣ The final product obtained when hydrogen bromide (HBr) is added to an ethyne molecule:

(a) 1,1-dibromoethane

(b) 1,1,2,2-tetrabromoethane

(c) 1,2-dibromoethane

(d) Bromoethane

All akynes can add two molecules of attacking reagents. The addition of first molecule of unsymmetrical reagent like HBr may or may not take place in accordance with Markownikov’s rule but the addition of second molecule of unsymmetrical reagent is always almost dictated by Markownikov’s rule. 

Ethyne being a symmetrical alkyne adds two molecules of HBr first forming chloroethene or vinyl bromide and finally 1,1-dibromoethane in accordance with Markownikov’s rule.

2️⃣ The formula of saturated hydrocarbon is C₃H₆, it should be

(a) Propane

(b) Propene

(c) Propyne

(d) Cyclopropane

C₃H₆ has two fewer hydrogens than saturated alkane propane (C₃H₈), indicating unsaturation or ring formation. The cyclic saturated hydrocarbons called cycloalkanes are expressed the general formula CₙH₂ₙ (n = 3,4,5,6 etc.). Since C₃H₆ obeys the general formula of cycloalkanes, hence it should be cyclopropane.

3️⃣ Ozonide on heating with zinc dust produces

(a) Alcohol

(b) Aldehyde

(c) Alkene

(d) Ether

Alkenes can be ozonolyzed to give an ozonide which finally decomposes to give aldehydes and ketones. This reaction involves the breaking of C=C of alkene by the use of ozone and reducing agent such as zinc. The ozonolysis of an alkene initially gives an ozonide intermediate which are unstable undergoes decomposition (either with Zn or H₂O₂) to form an aldehyde or ketone.

4️⃣ Which of the following pairs of compounds represents functional group isomerism?

(a) 1-butene and 2-butene

(b) Ethanol and dimethyl ether

(c) n-butane and isobutene

(d) diethyl ketone and methyl propyl ketone

Ethanol and dimethyl ether both have same molecular formula but different structural formulae due to the presence of different functional groups.

5️⃣ The substituent that can act as a meta director is

(a) –Cl

(b) –CH₃

(c) –OH

(d) –COOH

For meta directors groups, a less electronegative elements is directly attached the ring. Electron withdrawing groups are meta directing group. Usually carbon containing functional groups acts as a meta director. e.g. –NO₂, –CN, –CF₃, –COOH, –CHO, –COR, –COOR etc. 

For ortho-para directing groups a more electronegative element is directly attached to the ring. Electron releasing groups are ortho and para directing group. e.g. –X, –NH₃, –OH, –OCH₃ etc.

6️⃣ Welding gas among the following is

(a) Ethylene

(b) Acetylene

(c) Ethane

(d) Methane

Acetylene is called welding gas as it is used in welding and cutting metals.

7️⃣ Select the suitable chemical to distinguish between ethene and ethyne

(a) Alkaline KMnO₄

(b) Acidified KMnO₄

(c) Ammonical AgNO₃

(d) Bromine water

Suitable chemical to distinguish between Alkene (ethene) and 1-alkynes (ethyne) --- Ammonical AgNO₃

8️⃣ Acylation of benzene in the presence of AlCl₃ gives

(a) Toluene

(b) Acetophenone

(c) Phenol

(d) Xylene

Acylation of benzene in the presence of AlCl₃ gives Acetophenone refers to the Friedel–Crafts acylation, a classic electrophilic aromatic substitution reaction.

9️⃣ Meta directing group among the following is

(a) –OH

(b) –NH₂

(c) –CH₃

(d) –NO₂

Meta-directors are typically electron-withdrawing groups (EWGs). They pull electron density away from the benzene ring, making the meta position the most favorable for incoming electrophiles. If the group has a double/triple bond to O or N, or a positive charge, it's likely a meta-director. –NO₂ being a strong electron-withdrawing group (EWG) is a Meta directing group.     

1️⃣0️⃣ Benzene burns with smoky flame because of its:

(a) Inflammability

(b) High carbon %age

(c) High resonance energy

(d) Aromaticity

Benzene burns with a strong sooty yellow flame due to incomplete combustion resulting in the production of carbon particles or soot along with other byproducts like carbon monoxide and unburned benzene which is due to high percentage of carbon content or high carbon-hydrogen ratio. Details The flame temperature produced due to the combustion of aliphatic hydrocarbons with hydrogen-carbon (H/C) ratio of two or more (>2) is much higher than that of aromatic hydrocarbons with H/C ratio of 1 to 1.8. All hydrocarbon flames produce elemental carbon in the duration of the flame. The higher heat of the aliphatic flame makes all that carbon burn out completely to CO₂. Due to extra stability associated with aromatic compounds, they need comparatively higher temperature to react completely with oxygen producing aromatic flames and the flame temperature is not homogeneous, some parts are hotter, some are colder. In the colder part of the flame, the combustion of benzene will be incomplete and it does not burn out all the elemental carbon formed in the flame, so it passes through the flame as black soot of partly burned material.

1️⃣1️⃣ Which one of the following is/are correct regarding the physical states of alkanes? 

I. Alkanes consist of C1 to C4 are liquids 

II. Alkanes consist of C5 to C17 are waxy solids and 

 III. Alkanes above C17 are gases.:

(a) I only

(b) II and III only

(c) I, II and III

(d) None of them

Usually first few members of any class of organic compounds upto 5 carbons are gases, next ten are liquids are liquids and members more than 15 carbon atom solids.

1️⃣2️⃣ The boiling point of straight chain alkanes is ……….. than that of branched chain alkanes

(a) Higher

(b) Lower

(c) Equal

(d) Lesser

The boiling point of straight chain alkanes is higher than that of branched chain alkanes because straight chain alkanes are more extended occupying more surface area producing strong van der Waal’s forces. As the branching increases, surface area decreases which leads to weaker van der Waal’s forces thereby reducing boiling point of branched chain alkanes.

1️⃣3️⃣ In alkane, all the carbon atoms are

(a) sp³–hybridized

(b) sp²–hybridized

(c) sp–hybridized

(d) dsp³–hybridized

Since all the carbon atoms in alkanes are linked to four other atoms or groups, hence each carbon atoms in alkanes are sp³–hybridized.

1️⃣4️⃣ The C–C σ-bond in ethane is formed by the linear overlap of

(a) sp³–sp³

(b) sp²–sp²

(c) sp–sp

(d) s–sp³

Since each carbon atom in alkanes are sp³–hybridized, the C–C σ-bond in ethane is formed by the linear overlap of sp³–sp³

1️⃣5️⃣ The carbon atoms in cyclopropane are sp³-hybridized arranged in a …………. geometry

(a) Trigonal

(b) Tetrahedral

(c) Linear

(d) Angular

Cyclopropane is the smallest and simplest cycloalkanes with a ring composed of three carbon atoms and six hydrogen atoms. The carbon atoms in cyclopropane are sp³-hybridized arranged in a trigonal geometry with an internal angel of 60°, which is quite smaller than the bond angle (109.5°) exists in alkanes. In cyclopropane, each carbon atom is sp³-hybridized, which typically corresponds to a tetrahedral geometry with bond angles of approximately 109.5°. Despite the tetrahedral hybridization, the three-membered ring forces the carbon atoms into a triangle, with internal bond angles of 60°. This leads to angle strain because the ideal tetrahedral angle is much larger than 60°. So while the electronic geometry is tetrahedral due to sp³ hybridization, the molecular shape is distorted into a planar triangle.                     Summary:                                                              Hybridization basis: sp³ → tetrahedral                                                    Actual ring shape: strained triangle due to ring constraints

1️⃣6️⃣ Which is the smallest and simplest cycloalkanes?

(a) Cyclobutane

(b) Cycloethane

(c) Cyclopentane

(d) Cyclopropane

We need at least three carbons to construct a homocyclic ring, hence the smallest and simplest cycloalkanes will be cyclopropane (C₃H₆).

1️⃣7️⃣ Cycloalkanes are more stable than alkanes due to their

(a) sp³ hybridization

(b) Ring structure

(c) Tetrahedral structure

(d) strong IMF

Cycloalkanes are more stable than alkanes due to their ring structure. The carbon-carbon bond in the ring is more difficult to break than in open chain alkanes. Smaller rings like cyclopropane and cyclobutane are less stable due to significant angle strain.

1️⃣8️⃣ The pKₐ value of ethyne is

(a) 50

(b) 44

(c) 25

(d) 10

The pKₐ value of ethyne is 25 which shows that its acidic nature. Ethyne is a terminal alkyne. The hydrogen attached to the sp-hybridized carbon is relatively acidic compared to alkenes and alkanes. This is due to the high s-character (50%) of the sp orbital, which stabilizes the conjugate base (acetylide anion).

1️⃣9️⃣ The internal angle in cyclopropane is

(a) 90°

(b) 109.5°

(c) 104.5°

(d) 60°

The bond angle in cyclopropane is 60°, deviates significantly from the optimal angle of 109.5°, so it has very high angle strains. The sp³-sp³ orbitals can only overlap partially because of the angle deviation, so the overlapping is not as effective as it should be, and as a result, the C-C bond in cyclopropane is relatively weak.

2️⃣0️⃣Which reagent is used for the decarboxylation of sodium alkanoate?

(a) Soda lime

(b) Tollen’s reagent

(c) Soda ash

(d) None of them

The reagent used for the decarboxylation of sodium alkanoate into lower previous alkane is Soda lime which is a mixture of quick lime; CaO and concentrated NaOH solution. CaO is used to keep NaOH dry as NaOH tends to absorb moisture from air.

21 Ethyne gives ethane on complete hydrogenation adding ………. molecules of hydrogen gas.

(a) 1

(b) 2

(c) 3

(d) 4

Alkynes undergo electrophilic addition reactions adding two molecules of any attacking reagent. However addition of water and HCN to alkynes being nucleophilic addition proceeding through addition of only one molecule of these reagents.

22 Which alkane is produced by the decarboxylation of sodium propanoate?

(a) Methane

(b) Ethane

(c) Propane

(d) Butane

The decarboxylation of sodium alkanoate or sodium carboxylate on heating with soda lime gives lower previous alkane containing one carbon atom less than the corresponding acid salt. The new hydrogen atom in alkane is derived from the NaOH of soda lime. Decarboxylation of sodium propanoate gives ethane. Decarboxylation of sodium ethanoate gives methane.

23 The Catalytic Hydrogenation of Alkenes into alkane is known as

(a) Wurtz reaction

(b) Sabatier-Senderens Reactions

(c) Clemmensen reduction

(d) None of them

The Catalytic Hydrogenation of Alkenes into alkane is known as Sabatier-Senderens Reactions. Sabatier-Senderens reaction is the process of nickel-based hydrogenation and converting the unsaturated hydrocarbons into the saturated hydrocarbons by passing through their vapour and hydrogen over hot Raney Ni solid particles (fine-grained solid particle extracted from Ni-Al alloy).

24 Grignard’s Reagent is chemically called

(a) Tetraethyl lead

(b) Methylcobalamine

(c) Alkyl magnesium halide

(d) Dimethyl zinc

Alkyl magnesium halide is the chemical name of Grignard’s Reagent which is an organo-Mg compounds.

25 Two sequential addition reactions take place in

(a) Alkenes

(b) Alkanes

(c) Alkynes

(d) Benzene

Two sequential addition reactions takes place in alkyne. Addition of one equivalent of reagent forms an alkene, which can then add a second equivalent of reagent to yield a saturated product having four new bonds.

26 The metal used to prepare Grignard’s Reagent belongs to which group of the periodic table?

(a) IA

(b) IIA

(c) IIB

(d) VIIIB

Grignard’s Reagent contains Mg metal which belongs to IIA group of the periodic table.

27 Which one of the following is an organo-metallic compound?

(a) Haemoglobin

(b) Grignard’s Reagent

(c) Chlorophyll

(d) All of them

The most common organo metallic compounds are given below:

1. Grignard’s reagent; a highly reactive organic compound used in organic synthesis.

2. Alkyl lithium or Super Grignard reagent (R-Li)

3. Dialkyl zinc (R₂Zn) like Dimethyl zinc; used as insecticide

4. Tetraethyl lead; used as knock inhibitor in petroleum industry

5. Methylcobalamine; a derivative of vitamin B-12

6. Gilman reagent (R₂CuLi)

7. Dialkyl Cadmium (R₂Cd)

8. Haemoglobin (protein made up of four subunits each containing a heme group with an Fe that binds to oxygen.

9. Chlorophyll (Chlorophyll is a large molecules with prophyrin ring structure containing a magnesium atom)

28 Grignard’s reagent reacts with water in acidic medium to yield

(a) Alkenes

(b) Alcohol

(c) Carboxylic acid

(d) Alkanes

The acidified hydrolysis of Grignard’s reagent with water proceeds through nucleophilic substitution (SN) producing corresponding alkanes.

29 Grignard’s reagent reacts with ………….. to produce an alkane

(a) Benzene

(b) primary amide

(c) tertiary amine

(d) primary amine

Grignard’s reagent also undergoes nucleophilic substitution (SN) with compounds containing terminal polarized acidic hydrogen such as ammonia, primary amines and secondary amines producing corresponding alkanes. TERTIARY AMINES CANNOT PRODUCE ALKANES FROM GRIGNARD’S REAGENT DUE TO LACK OF TERMINAL HYDROGEN.

RMgX + RNH₂ → RH + Mg(NHR)X

30 Which compound is used as solvent during the synthesis of Grignard’s Reagent?

(a) Ether

(b) Alcohol

(c) Acetic acid

(d) Water

Grignard’s Reagent chemically called organo magnesium halides or alkyl magnesium halide is an organic-metallic compound commonly formularized as R–MgX, prepared by the action of Alkyl Halides on Mg-turnings (powder) kept in dry ether as a solvent (any water present reacts with Grignard’s Reagent to yield alkane).

The special properties of ethers (polarity, lone pairs, but relatively unreactive) enhance the formation and use of many organo-metallic reagents. For example, Grignard reagents cannot form unless an ether is present, possibly to share its lone pairs of electrons with the magnesium atom. This sharing of electrons stabilizes the reagent and helps keep it in solution.