Lab 1 Lewis Structures & Structural Formulas

Purpose

• In this exercise we will investigate Lewis Structures, Structural Formulas and Condensed Structural Formulas, how to draw them, how to interpret them and how 2D and 3D representations of chemicals look different or similar.

• We will also be introduced to the concept of isomers.

Learning objectives are:

• Convert Lewis Structures to a chemical formula and vice versa.

• Convert Structural Formulas to a chemical formula and vice versa.

• Convert Structural Formulas to Condensed Structural formulas and vice versa.

• Distinguish between Structural Formulas and Lewis Structures.

• Predict what constitutional isomers are possible for a compound with a given chemical formula.

• Differentiate between a 2D representation of a molecule and the actually 3D arrangement of atoms in the molecule.

Knowledge

This assignment will help you review & strengthen knowledge on the following content:

• Common bond number

• How to draw Lewis Structures

• Valence electrons and VSEPR (Valence Shell Electron Pair Repulsion) theory

Skills

After this assignment you should be able to:

1. Differentiate between Lewis Structures, Structural Formulas & Condensed Structural Formulas.

2. Make predictions about what isomers of a chemical formula will form.

3. Interpret 2D chemical formulas to determine what their 3D shape should be based on VSEPR (Valence Shell Electron Pair Repulsion) theory

Criteria for Passing Grade

In order to achieve a passing grade student must do the following as a minimum:

• Make detailed observations during the completion of the procedure above.

• Answer all of the questions using detailed, complete answers with clear reasoning and evidence used as necessary to support your answer.

• Complete all answers in their own words.

• It is recommended to compare your lab results with other lab teams. Just be sure all your answers are your own original thoughts and in your own words.

Introduction

The study of organic chemistry involves almost exclusively molecular substances, i.e., compounds in which the bonds connecting the atoms are covalent. In a covalent bond one or more electron pairs are shared by two atoms. To show covalent bonds in a molecule, begin with the electron dot structures for the individual atoms and then put all the atoms together so that each atom achieves a completely filled outer shell. Remember that a completely filled outer shell for hydrogen contains 2 electrons and for most other elements contains 8 electrons.

Lewis Structures

Example: Draw a Lewis structure for water H2O. Note that H2O is sometimes referred to as a molecular formula.

The correct arrangement of atoms in the Lewis Structure is:

Lone pairs are shown with 2 dots, shared electron pairs are shown as a line connecting the two atoms. Double bonds are shown with two lines connecting the atoms in the bond. Triple bonds are shown with two lines connecting the atoms in the bond.

Structural Formulas

Structural formulas are similar to Lewis structures, but omit the lone pairs. See example of structural formula below for water.

Lewis structures are not used that often in organic chemistry, but structural formulas are more commonly used. Fortunately, there is an easy way to draw structural formulas without first drawing a Lewis structure. Most of the elements in organic compounds consistently form the same number of bonds in neutral compounds. The following table gives the normal number of bonds for some of the most common elements found in organic compounds (note there are exceptions to this general pattern):

Double & Triple Bonds

So far all of the covalent bonds have been single bonds. As you probably remember, we can also have double and triple covalent bonds. For example, draw the Lewis structure for an oxygen molecule, O2. The two oxygen atoms can form one covalent bond as follows:

However, each atom now has only 7 electrons in the outer shell. So, another pair must be shared:

Now each oxygen atom has 8 electrons in the outer shell. This is known as a double bond, consisting of two shared pairs of electrons. The structural formula looks like this:

Note that this structure is consistent with our table for the normal number of covalent bonds for oxygen. We can use the same table for drawing structural formulas for compounds that have double and triple bonds.

Condensed Structural Formulas

A condensed structural formula is a system of writing organic structures in a line of text. It shows all atoms, but omits the vertical bonds and most or all the horizontal bonds. The condensed structural formulas of ethane, propane, and ethanol are: CH₃CH₃, CH₃CH₂CH₃, and CH₃CH₂OH

It uses parentheses to show that polyatomic groups within a formula are attached to the nearest non-hydrogen atom on the left. So, the condensed structural formula of propan-2-ol (or 2-propanol) is CH₃CH(OH)CH₃.It also uses parentheses to show that polyatomic groups at the beginning of a formula are attached to atom on the right. So, we could also write the condensed structural formula of propan-2-ol as (CH₃)₂CHOH.

And we could write the condensed structural formula of hexane as CH₃CH₂CH₂CH₂CH₂CH₃ or CH₃(CH₂)₄CH₃.

Molecular Models

Now let's extend our study of organic molecular structures to the use of molecular models. The following table gives the color of the balls corresponding to each element. You might want to check to see that the number of holes in each colored ball corresponds to the number of bonds that each of the elements normally forms.

REPORT – Lewis Structures & Structural Formulas

DUE: Name:

Due at the beginning of the next lab for credit! Complete pre-lab in Canvas prior to lab.

Exercise l: Draw the Lewis structure and structural formulas for the following:

Exercise 2: Draw structural formulas for the following compounds:

1.CH3Cl 2. C2H6

3. C3H8 4. C2H7N

Exercise 3: Draw structural formulas for the following:

N2

C2H2

C2H4

Exercise 4:

• Make a model of a hydrogen molecule H2. (Use the shorter gray bonds for single bonds.)

• Now draw the structural formula for H2 below.

• Does the 2D structural formula you drew have the same shape as the model? Fill in your answer below, provide an explanation for your answer.

Exercise 5:

• Make a model of a methane molecule CH4. (Use the shorter gray bonds for single bonds.)

• Now draw the structural formula for CH2 below.

• Does the 2D structural formula you drew have the same shape as the model? Fill in your answer below, provide an explanation for your answer.

NOTE: Most likely you answered no above (it's OK if you said yes). Methane is not a flat molecule, as the structural formula seems to indicate. The structural formulas we have been drawing usually do not accurately represent the actual three-dimensional shape of the molecules. This is OK as long as we are aware of this limitation. Later we will talk more about the actual shape.

Exercise 6:

• Make a model of methanol, CH3OH.

• Draw the structural formula for methanol. Do they look identical in shape? Fill in your answer below, provide an explanation for your answer.

NOTE: We could have written the molecular formula for methanol as CH4O. Writing it as CH3OH makes it more clear what the actual bonding is. So, often molecular formulas will be written in a more clear way like this.

Exercise 7:

1. Make a model of propane (C3H8) then draw the condensed structural formula.

2. Make a model of ethylene (C2H4). Here you will need to use the longer, more flexible gray bonds for the double bond. Ask for help if you're not sure how to do this. Draw the condensed structural formula.

3. Make a model of C2H6O. Now make another one that is structurally different from the first one! Draw the condensed structural formulas for each.

NOTE: The structures in part C are examples of isomers, compounds with the same molecular formula but different structures. If you must break and make new bonds in order to convert one structure into the other, then they are isomers

Exercise 8: Take one of the models of C2H6O. Are there any ways that you can change its VSEPR shape without breaking any bonds? Fill in your answer below, provide an explanation for your answer.

Exercise 9: Draw structural formulas for all of the structural isomers of C4H10.

Hint: There are two. You may find it helpful to build models to solve this exercise.

Exercise 10: Draw structural formulas for all of the structural isomers of C5H12.

Hint: There are three.

Exercise 11: Draw structural formulas for all of the isomers of C4H8. Hint: There are six. Using models can be very helpful with this one.