Problem 70 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) What is the empirical formula of starch? (b) What is the monomer that forms the basis of the starch polymer? (c) What bond connects the monomer units in starch: amide, acid, ether, ester, or alcohol?

Step-by-Step Solution

Answer

(a) The empirical formula of starch is \(\displaystyle C_{6} H_{10} O_{5}\). (b) The monomer that forms the basis of the starch polymer is glucose, with a molecular formula of \(\displaystyle C_{6} H_{12} O_{6}\). (c) The bond connecting the monomer units in starch is an ether bond, also known as a glycosidic bond.

Step 1: (a) Empirical formula of starch

Starch is a complex carbohydrate, mainly found in plants. It consists of two types of glucose polymers, amylose and amylopectin. Both amylose and amylopectin consist of glucose monomers connected through glycosidic bonds. The empirical formula of glucose is \(\displaystyle C_{6} H_{10} O_{5}\).

Step 2: (b) Monomer forming the basis of the starch polymer

The monomer that forms the basis of starch is the glucose molecule. Glucose is a simple sugar with the molecular formula of \(\displaystyle C_{6} H_{12} O_{6}\).

Step 3: (c) The bond connecting the monomer units in starch

The bond that connects glucose molecules in starch is called a glycosidic bond. A glycosidic bond consists of an oxygen atom connecting two carbon atoms, one from each monomer. This type of bond is an ether bond, so the correct answer is an ether bond.

Key Concepts

Empirical Formula

Understanding the empirical formula of starch helps in grasping its basic composition. Starch is a type of carbohydrate and its structural units are glucose molecules. This makes it a polysaccharide, which is a large molecule formed by the combination of many smaller sugar molecules. The empirical formula represents the simplest whole number ratio between the elements in a compound. For starch, the empirical formula is based on its glucose composition.

• The empirical formula of glucose, which is the building block of starch, is \(C_6H_{10}O_5\).
• This formula shows the proportion of carbon, hydrogen, and oxygen atoms in glucose.

It's important to note that while the molecular formula for glucose is \(C_6H_{12}O_6\), the empirical formula is different because it simplifies the ratio of atoms. In the process of forming starch, glucose loses a molecule of water, resulting in the empirical formula \(C_6H_{10}O_5\).

Glucose Monomer

The fundamental building block of starch is the glucose monomer. A monomer is a molecule that can undergo polymerization, binding to other molecules to form a large, multi-unit structure. In the case of starch, glucose is the singular unit that repeats within the polymer chain.

• The molecular formula of a glucose molecule is \(C_6H_{12}O_6\).
• Glucose is a simple sugar and plays a vital role in biological energy transfer.

When multiple glucose monomers join together, they form the polysaccharides known as amylose and amylopectin, which are the two types of starch. Glucose's structure allows it to link repeatedly, forming complex carbohydrates essential in plant energy storage.

Glycosidic Bond

A crucial feature in the formation of starch from glucose is the glycosidic bond. This is a type of covalent bond that connects two sugar molecules, resulting in a disaccharide and further leading to polysaccharides like starch through a series of such bonds.

• A glycosidic bond involves an oxygen atom linking the carbon atoms of two glucose molecules.
• In starch, this bond is specifically an ether bond.

The glycosidic bond is significant because it determines the structure and digestibility of starch. These bonds can form in different orientations, such as alpha and beta links, which affects the polymer's properties. In the case of starch, the alpha links cause the polymer chains to coil into helical structures, resulting in more easily digestible energy sources for humans.