Carbohydrates Questions in English

(C) The structure of $\beta-D-(+)$-glucose is a cyclic hemiacetal form of glucose.
In the cyclic structure,the carbon atoms at positions $C_1, C_2, C_3, C_4,$ and $C_5$ are chiral centers.
$C_1$ is the anomeric carbon,which is bonded to $-H, -OH, -O-$ (ring oxygen),and $-C_2$.
$C_2, C_3,$ and $C_4$ are bonded to four different groups.
$C_5$ is bonded to $-H, -CH_2OH, -O-$ (ring oxygen),and $-C_4$.
Therefore,there are $5$ chiral carbon atoms in $\beta-D-(+)$-glucose.

(D) Starch and cellulose are polysaccharides,which are complex carbohydrates. They are polymers formed by the condensation of many monosaccharide units,which are commonly known as simple sugars (specifically $D$-glucose units).

(D) The reaction of glucose with bromine water $(Br_2/H_2O)$ acts as a mild oxidizing agent that oxidizes the aldehyde group $(-CHO)$ to a carboxylic acid group $(-COOH)$ while leaving the secondary alcoholic groups unaffected.
$C_6H_{12}O_6 + [O] \xrightarrow{Br_2/H_2O} CH_2OH(CHOH)_4COOH$
Thus,the product formed is gluconic acid.

(B) Glucose is a reducing sugar because it contains an aldehyde group. $ \text{When heated with Fehling's solution, it reduces } Cu^{2+} \text{ ions to } Cu^{+} \text{ ions, forming a red precipitate of cuprous oxide } (Cu_2O). $
$ \text{Reaction: } C_6H_{12}O_6 + 2Cu^{2+} + 5OH^- \rightarrow C_6H_{11}O_7^- + Cu_2O (\text{red ppt}) + 3H_2O. $

(B) Glucose is a reducing sugar because it contains an aldehyde group $(-CHO)$.
In the process of silvering mirrors,glucose is used to reduce Tollens' reagent $([Ag(NH_3)_2]^+)$ to metallic silver $(Ag)$,which deposits on the glass surface to form a mirror.
Therefore,glucose is the correct answer.

(D) Tollens' reagent is reduced by all sugars that contain a free aldehyde group or those that can isomerize to form an aldehyde group in an alkaline medium.
$1$. Glucose is an aldose and contains a free aldehyde group,so it reduces Tollens' reagent.
$2$. Fructose,although a ketose,undergoes tautomerization in the alkaline medium of Tollens' reagent to form an aldose,thus it also reduces Tollens' reagent.
$3$. Lactose is a reducing sugar because it contains a free hemiacetal group,which allows it to exist in equilibrium with its open-chain aldehyde form.
Therefore,all the given options reduce Tollens' reagent.

(D) The $D$ and $L$ notations are used to describe the configuration of the chiral carbon atom furthest from the carbonyl group,relative to the reference molecule $glyceraldehyde$. In $D-glucose$,the $D-$ prefix indicates that the hydroxyl group $(-OH)$ on the chiral carbon furthest from the aldehyde group is on the right side in the Fischer projection,which corresponds to the configuration of $D-glyceraldehyde$.

(A) Carbohydrates are classified based on their behavior upon hydrolysis.
Monosaccharides are the simplest form of carbohydrates and cannot be hydrolyzed further into smaller units of polyhydroxy aldehydes or ketones.
Examples include $Glucose$ and $Fructose$.

(A) Sucrose is a disaccharide formed by the condensation of one molecule of $\alpha - D - \text{glucose}$ (specifically $\alpha - D - \text{glucopyranose}$) and one molecule of $\beta - D - \text{fructose}$ (specifically $\beta - D - \text{fructofuranose}$).
The glycosidic linkage is formed between $C1$ of $\alpha - D - \text{glucopyranose}$ and $C2$ of $\beta - D - \text{fructofuranose}$.

(B) $\alpha-$ and $\beta-$ glucose are stereoisomers that differ in configuration only at the $C-1$ carbon atom,which is the anomeric carbon. Such isomers are specifically known as anomers.

(C) Milk contains a disaccharide called lactose. During digestion,lactose undergoes hydrolysis in the presence of the enzyme lactase to produce two monosaccharides: $Glucose$ and $Galactose$. The reaction is: $C_{12}H_{22}O_{11} (Lactose) + H_2O \rightarrow C_6H_{12}O_6 (Glucose) + C_6H_{12}O_6 (Galactose)$.

(D) Sucrose $(C_{12}H_{22}O_{11})$ undergoes dehydration when heated with concentrated $H_2SO_4$.
The reaction is: $C_{12}H_{22}O_{11} \xrightarrow{conc. H_2SO_4} 12C_{(s)} + 11H_2O_{(l)}$.
Concentrated $H_2SO_4$ acts as a strong dehydrating agent,removing water molecules from the sugar,leaving behind carbon (charcoal).

(D) Anomers are a type of stereoisomer found in carbohydrates that differ in configuration only at the anomeric carbon (the carbon derived from the carbonyl carbon in the open-chain form).
In cyclic structures,this is the carbon attached to two oxygen atoms.
Option $D$ shows two cyclic forms of glucose where the configuration at the $C-1$ (anomeric) carbon is different (one has $-OH$ on the right,the other on the left),while all other chiral centers remain identical.
Therefore,these represent $\alpha-D-glucose$ and $\beta-D-glucose$,which are anomers.

(C) Starch is a polysaccharide composed of two components: amylose and amylopectin.
Both components are polymers of $\alpha-D$-glucose units.
In amylose,the glucose units are linked by $\alpha$-glycosidic linkages between $C-1$ of one glucose unit and $C-4$ of the next glucose unit.
Similarly,in the linear chains of amylopectin,the glucose units are linked by $\alpha-1,4$-glycosidic linkages.

(B) Invert sugar is an equimolar mixture of $D(+)$-glucose and $D(-)$-fructose.
It is obtained by the hydrolysis of sucrose.
The hydrolysis of sucrose results in a change in the sign of optical rotation from dextrorotatory $(+)$ to levorotatory $(-)$,which is why it is called invert sugar.

(C) Cellulose is a linear polysaccharide of $D$-glucose units joined by $\beta$-glycosidic linkages. It is the main structural component of plant cell walls,providing rigidity and strength to the plant.

(C) The reaction of glucose with Tollen's reagent is a characteristic test for the presence of an aldehydic group $(-CHO)$.
Glucose gets oxidized to gluconic acid,and the silver ions $(Ag^+)$ are reduced to metallic silver $(Ag)$,which forms a silver mirror on the inner walls of the test tube.
Reaction: $C_6H_{12}O_6 + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow C_6H_{11}O_7^- + 2Ag + 4NH_3 + 2H_2O$.

(B) Glucose is an aldohexose containing an aldehyde group $(-CHO)$,whereas fructose is a ketohexose containing a ketone group $(-C=O)$.
Because of the presence of the aldehyde group,glucose acts as a reducing sugar and gives a positive silver mirror test with Tollen's reagent.
Fructose,although a ketose,also gives a positive test due to its isomerization into glucose and mannose in the alkaline medium of the reagent,but glucose is specifically characterized by its aldehyde functionality which is the primary structural difference.

(D) Glycolysis is a metabolic pathway that occurs in the cytoplasm of the cell.
It involves the breakdown of one molecule of $Glucose$ $(C_6H_{12}O_6)$ into two molecules of $Pyruvate$ $(CH_3COCOOH)$ through a series of enzyme-catalyzed reactions.
This process generates a net gain of $2$ $ATP$ and $2$ $NADH$ molecules.

(C) Lactose is a disaccharide composed of two monosaccharide units: $\beta-D-glucose$ and $\beta-D-galactose$.
The hydrolysis reaction is: $\text{Lactose} \xrightarrow{H_2O} \text{Glucose} + \text{Galactose}$.

(B) Milk contains $Lactose$,which is a disaccharide composed of $Glucose$ and $Galactose$ units.

(B) Glucose reacts with excess phenylhydrazine $(C_6H_5NHNH_2)$ to form glucosazone,which is a crystalline osazone derivative.
This reaction involves the oxidation of the $C-2$ carbon and the formation of a hydrazone linkage at both $C-1$ and $C-2$ positions.
The chemical equation is: $CHO(CHOH)_4CH_2OH + 3C_6H_5NHNH_2 \rightarrow C_6H_{10}O_4(NNHC_6H_5)_2 + C_6H_5NH_2 + NH_3 + 2H_2O$.

(B) The hydrolysis of sucrose $(C_{12}H_{22}O_{11})$ in the presence of acid or the enzyme invertase produces an equimolar mixture of $D-(+)$-glucose and $D-(-)$-fructose.
This process is called the inversion of cane sugar because the optical rotation changes from dextrorotatory $(+66.5^{\circ})$ to levorotatory $(-39.9^{\circ})$.

(A) Humans lack the enzyme $cellulase$ required to break down the $\beta$-glycosidic linkages present in $cellulose$. Therefore,$cellulose$ cannot be digested by the human digestive system.

(C) When glucose reacts with one equivalent of phenylhydrazine $(C_6H_5-NH.NH_2)$,it forms glucose phenylhydrazone.
However,when glucose reacts with an excess of phenylhydrazine,it undergoes a series of reactions to form a di-phenylhydrazone derivative known as glucosazone.
The reaction involves the oxidation of the $C-2$ carbon and the formation of a hydrazone linkage at both $C-1$ and $C-2$ positions.

(C) Invert sugar is an equimolar mixture of $D-(+)-glucose$ and $D-(-)-fructose$.
Since the specific rotation of $D-(-)-fructose$ $(-92.4^{\circ})$ is greater in magnitude than that of $D-(+)-glucose$ $(+52.7^{\circ})$,the resulting mixture has a net negative specific rotation.
Therefore,invert sugar is laevorotatory.

(C) Mutarotation is a property exhibited by reducing sugars that have a free hemiacetal or hemiketal group,allowing them to exist in equilibrium between their $\alpha$ and $\beta$ anomeric forms in solution.
Sucrose is a non-reducing sugar because the glycosidic linkage is formed between the anomeric carbons of both glucose and fructose ($C_1$ of glucose and $C_2$ of fructose).
Since there is no free hemiacetal or hemiketal group available,sucrose cannot undergo mutarotation.
Therefore,the correct option is $(C)$.

(C) Glucose is an aldohexose containing an aldehyde group $(-CHO)$.
Fructose is a ketohexose containing a ketone group $(>C=O)$.
Since they have the same molecular formula $(C_6H_{12}O_6)$ but different functional groups,they are functional isomers.

(C) In structure $(I)$,the linkage $(A)$ connects the $C_1$ carbon of one glucose unit to the $C_4$ carbon of another glucose unit,representing a $C_1-C_4$ glycosidic linkage.
In structure $(II)$,the linkage $(B)$ connects the $C_1$ carbon of one glucose unit to the $C_6$ carbon (part of the $CH_2OH$ group) of another glucose unit,representing a $C_1-C_6$ glycosidic linkage.
In structure $(III)$,the linkage $(C)$ connects the $C_1$ carbon of one glucose unit to the $C_4$ carbon of another glucose unit,representing a $C_1-C_4$ glycosidic linkage.
Therefore,$(A)$ and $(C)$ are $C_1-C_4$ linkages,and $(B)$ is a $C_1-C_6$ linkage.

(D) Anomers are a specific type of epimers that differ in configuration only at the anomeric carbon atom,which is $C-1$ in the case of aldoses like glucose.
Therefore,$\alpha-D$-glucose and $\beta-D$-glucose are anomers because they differ in the configuration of the hydroxyl group at the $C-1$ position.

(B) When $D$-glucose forms a cyclic structure (pyranose ring),a new chiral center is created at the $C-1$ carbon atom.
This results in two isomeric forms known as $\alpha-D$-glucopyranose and $\beta-D$-glucopyranose.
These isomers,which differ only in the configuration at the anomeric carbon $(C-1)$,are called anomers.

(C) Sucrose is a disaccharide composed of one molecule of $\alpha-D-glucose$ and one molecule of $\beta-D-fructose$ linked by a glycosidic linkage.
Upon hydrolysis,sucrose breaks down into its constituent monosaccharides:
$C_{12}H_{22}O_{11} + H_2O \xrightarrow{H^+} C_6H_{12}O_6 (\text{glucose}) + C_6H_{12}O_6 (\text{fructose})$
Therefore,the hydrolysis of sucrose yields one molecule each of glucose and fructose.

(B) Invert sugar is an equimolar mixture of $D-(+)-glucose$ and $D-(-)-fructose$.
It is obtained by the hydrolysis of sucrose.
Sucrose is dextrorotatory $(+66.5^{\circ})$,but upon hydrolysis,it yields a mixture that is levorotatory because the specific rotation of $D-(-)-fructose$ $(-92.4^{\circ})$ is greater than that of $D-(+)-glucose$ $(+52.7^{\circ})$.

(C) $\alpha - D - (+) -$ glucose and $\beta - D - (+) -$ glucose differ only in the configuration of the hydroxyl group at the $C-1$ carbon atom,which is the anomeric carbon.
Such isomers,which differ in configuration at the hemiacetal or hemiketal carbon,are known as anomers.

(D) The given compound is an aldohexose (specifically glucose or its stereoisomers) with the structure $CHO-(CHOH)_4-CH_2OH$.
There are $n = 4$ chiral carbon atoms in this molecule.
The number of optically active isomers $(a)$ is given by $2^n = 2^4 = 16$.
Since the molecule is asymmetric (the two ends are different,$CHO$ and $CH_2OH$),there are no meso compounds $(m = 0)$.
Therefore,the total number of optical isomers is $2^n = 16$.

(C) The open-chain structure of $D$-glucose contains $4$ chiral carbon atoms.
According to the formula for the number of stereoisomers,$N = 2^n$,where $n$ is the number of chiral centers.
Therefore,$N = 2^4 = 16$.

(C) Sucrose is a disaccharide,not a dipeptide. Aspartame is a dipeptide formed from aspartic acid and phenylalanine. Therefore,the statement in option $C$ is incorrect.

(C) The hydrolysis of sucrose yields an equimolar mixture of $D-(+)$-glucose and $D-(-)$-fructose. Since the sign of optical rotation changes from dextrorotatory $(+66.5^{\circ})$ to levorotatory $(-39.9^{\circ})$,this process is known as the inversion of sucrose,and the resulting mixture is called invert sugar.

(C) The mild oxidation of glycerol with $H_2O_2/FeSO_4$ (Fenton's reagent) yields a mixture of glyceraldehyde and dihydroxyacetone,which is collectively known as glycerose.
$CH_2OH-CHOH-CH_2OH \xrightarrow{H_2O_2/FeSO_4} CH_2OH-CHOH-CHO + CH_2OH-CO-CH_2OH$
(Glyceraldehyde + Dihydroxyacetone = Glycerose).

(A) Mutarotation is a property exhibited by reducing sugars that possess a free hemiacetal or hemiketal group,allowing them to exist in equilibrium between their $\alpha$ and $\beta$ anomeric forms in solution.
$A$. Sucrose is a non-reducing sugar because its glycosidic linkage is formed between the anomeric carbons of both glucose and fructose units,leaving no free hemiacetal or hemiketal group.
Therefore,sucrose does not show mutarotation.

(D) Glucose exists in a cyclic hemiacetal form in the solid state,but in aqueous solution,it exists in equilibrium with its open-chain aldehyde form. This open-chain form contains a free $-CHO$ group,which allows it to undergo characteristic reactions of aldehydes.

(B) The correct answer is $B$.
Benedict's solution is used for the quantitative determination of glucose in blood because it reacts with the aldehyde group of glucose to form a red precipitate of $Cu_2O$,which can be measured.
Tollen's reagent is typically used for qualitative detection of aldehydes,while Benedict's solution is preferred for quantitative analysis in clinical settings.

(C) When glucose reacts with methyl alcohol in the presence of dry $HCl$ gas,it forms a mixture of two isomeric methyl glucosides,namely $\alpha$-methyl glucoside and $\beta$-methyl glucoside.
This reaction involves the formation of an acetal at the anomeric carbon $(C-1)$.

(B) $Molisch's$ test is a general test for the detection of carbohydrates.
It involves the reaction of carbohydrates with $\alpha$-naphthol in the presence of concentrated $H_2SO_4$,which results in the formation of a purple or violet colored ring at the junction of the two liquids.

(D) In a Fischer projection,the $D/L$ configuration is determined by the $-OH$ group on the chiral carbon furthest from the aldehyde group (the lowest chiral carbon).
If the $-OH$ is on the right,it is $D$; if on the left,it is $L$.
$(1)$ $D$-erythrose: Both $-OH$ groups are on the same side (right).
$(2)$ $D$-threose: $-OH$ groups are on opposite sides,with the bottom $-OH$ on the right.
$(3)$ $L$-erythrose: Both $-OH$ groups are on the same side (left).
$(4)$ $L$-threose: $-OH$ groups are on opposite sides,with the bottom $-OH$ on the left.
Thus,the correct order is $D$-erythrose,$D$-threose,$L$-erythrose,$L$-threose.

(B) Sucrose is a non-reducing sugar because it does not have a free $-CHO$ or ketonic group.

(D) $D(+)$-glucose contains an aldehyde group at $C1$.
When it reacts with hydroxylamine $(NH_2OH)$,the carbonyl oxygen is replaced by $=NOH$ to form an oxime.
The configuration of the chiral centers ($C2$ to $C5$) remains the same as in $D$-glucose ($C2-OH$ right,$C3-OH$ left,$C4-OH$ right,$C5-OH$ right).
The structure is: $CH=NOH - C(H)(OH) - C(OH)(H) - C(H)(OH) - C(H)(OH) - CH_2OH$.

(A) Sucrose is a disaccharide composed of $\alpha-D$-glucopyranose and $\beta-D$-fructofuranose units.
These units are linked by an $\alpha, \beta-1,2$-glycosidic linkage between the $C-1$ of the glucose unit and the $C-2$ of the fructose unit.