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Lesson Instructions
Lesson Instructions
Review the learning goals and success criteria for Topic 2: Alkenes and Alkynes.
Review the minds on, action, and consolidation sections of the lesson, completing any tasks as outlined.
Complete the fill-in-the-blanks handouts that have been provided to you. They have been linked below.
Practice and apply what you've learned by completing the practice tasks outlined at the end of the lesson.
Learning Goals
Learning Goals
I am learning to ...
Observe molecular models of organic molecules.
Name and draw alkenes and alkynes.
Name and draw stereoisomers for an alkene.
Minds On
Minds On
Molecular Models
Molecular Models
In a traditional setting, we can use molecular model kits to compare alkanes, alkenes, and alkynes. However, we will start this lesson by using virtual models to explore some of the properties of alkanes, alkenes, and alkynes.
ALKANES
ALKANES
We know that alkanes are hydrocarbons that contain only single bonds.
The general algebraic formula for an alkane is CnH2n+2.
Let's look at a virtual model of pentane (5-carbon parent chain, only single bonds). You can interact, touch, and rotate the virtual model.
Pentane
Pentane
Pentane appears as a clear colorless liquid with a petroleum-like odor. Less dense than water and insoluble in water. Hence floats on water. Vapors are heavier than air.
Formula - C5H12
Molecular Weight - 72.15 u
% Composition of C - 83.236%
% Composition of H - 16.764%
ALKENES
ALKENES
Alkenes are hydrocarbons that contain at least one double bond.
The general algebraic formula for an alkane is CnH2n.
If the 5 carbon alkane was called pentane, the 5 carbon alkene is called pentene.
Let's look at a virtual model of pentene (5-carbon parent chain, presence of a double bond). You can interact, touch, and rotate the virtual model.
Pentene
Pentene
Pent-1-ene is a colorless liquid with an odor of gasoline. Insoluble in water and less dense than water. Hence floats on water. Flammable vapor is produced.
Formula - C5H10
Molecular Weight - 70.13 u
% Composition of C - 85.628%
% Composition of H - 14.372%
ALKYNES
ALKYNES
Alkynes are hydrocarbons that contain at least one triple bond.
The general algebraic formula for an alkane is CnH2n-2.
If the 5 carbon alkane was called pentane, the 5 carbon alkene was called pentene, the 5 carbon alkyne is called pentyne.
Let's look at a virtual model of pentyne (5-carbon parent chain, presence of a triple bond). You can interact, touch, and rotate the virtual model.
Pentyne
Pentyne
Pent-1-yne has been used in preparation of lithium acetylides, required for asymmetric synthesis of α,α-dibranched propargyl sulfinamides.
Formula - C5H8
Molecular Weight - 68.12 u
% Composition of C - 88.162%
% Composition of H - 11.838%
You have now observed the structural difference between alkanes, alkenes, and alkynes. Practice drawing a line diagram for each of the models you have interacted with. When you are done, click here to see the correct drawings.
Action
Action
Alkenes
Alkenes
Alkenes contain at least one double bond.
Has a general formula of CnH2n.
Alkynes
Alkynes
Alkynes contain at least one triple bond.
Has a general formula of CnH2n-2.
Since alkenes and alkynes are NOT bonded to the maximum possible number of atoms, these compounds are often referred to as unsaturated hydrocarbons.
Saturated and unsaturated hydrocarbons provide the chemistry behind the saturated and unsaturated fats in our diet.
Naming Alkenes and Alkynes
Naming Alkenes and Alkynes
The parent chain must include double or triple bonds at the lowest position number.
The parent chain must include the double bond.
Alkenes end with -ene, and alkynes end with -yne.
Position # is shown in front of the ending.
If more than one double/triple bond exists, use prefixes (diene, triene, etc.).
When numbering the main chain, double and triple bonds have priority over alkyl groups.
When you are numbering the parent chain, the double and triple bond are the priority with respect to lowest position number, then alkyl groups.
Figure 1. In the diagram on the left, the parent chain is numbered so that the double bond comes from the first carbon, and the methyl group branching off of the third carbon. This method keeps the double bond in the lowest possible position number and keeps the methyl group in a low position number. In the diagram on the right, the double bond is not prioritized with respect to lowest position number and the resulting name for this compound would be incorrect.
Practice - Naming Alkenes and Alkynes
Practice - Naming Alkenes and Alkynes
Example 1. Name the following organic compound.
Example 1. Name the following organic compound.
Figure 2. 4-methylpent-2-ene.
Example 2. Name the following organic compound.
Example 2. Name the following organic compound.
Figure 3. 3-ethylhex-2-ene.
Example 3. Name the following organic compound.
Example 3. Name the following organic compound.
Figure 4. 3-propan-2-ylhexa-1,3-diene.3-isopropylhexa-1,3-diene.
Example 4. Name the following organic compound.
Example 4. Name the following organic compound.
Figure 5. 3-ethyl-4,4-dimethylpent-1-yne.
Example 5. Name the following organic compound.
Example 5. Name the following organic compound.
Figure 6. 3-ethyl-6-methyloct-4-yne.
Practice - Drawing Alkenes and Alkynes
Practice - Drawing Alkenes and Alkynes
Example 6. Draw the following organic compound.
Example 6. Draw the following organic compound.
3-methylpent-2-ene
3-methylpent-2-ene
Example 7. Draw the following organic compound.
Example 7. Draw the following organic compound.
2,5,7-trimethyl-5-propyloct-3-yne
2,5,7-trimethyl-5-propyloct-3-yne
Practice - Naming Cyclic Alkenes and Alkynes
Practice - Naming Cyclic Alkenes and Alkynes
Example 8. Name the following organic compound.
Example 8. Name the following organic compound.
Figure 7. 3,4-dimethylcyclopentene
Practice - Drawing Cyclic Alkenes and Alkynes
Practice - Drawing Cyclic Alkenes and Alkynes
Example 9. Draw the following organic compound.
Example 9. Draw the following organic compound.
3,3-diethyl-2-methylcyclopentene
3,3-diethyl-2-methylcyclopentene
Stereoisomers (Geometric Isomers)
Stereoisomers (Geometric Isomers)
Stereoisomers are pairs of molecules which have the same structure, but, whose functional groups are rotated into a different orientation in three-dimensional space.
cis STEREOISOMERS
cis STEREOISOMERS
cis means that the two groups are on the same side of the double bond.
Figure 8. Note, the position of the chlorine on the same side of the double bond.
trans STEREOISOMERS
trans STEREOISOMERS
trans means that the two groups are on the opposite side of the double bond.
Figure 9. Note the position of the chlorine on the opposite sides of the doubl bond.
Figure 10. Another example of cis- and trans- stereoisomers. Models of (left) cis-2-butene and (right) trans-2-butene.
Figure 11. Rotation about Bonds. In 1,2-dichloroethane (a), free rotation about the C–C bond allows the two structures to be interconverted by a twist of one end relative to the other. In 1,2-dichloroethene (b), restricted rotation about the double bond means that the relative positions of substituent groups above or below the double bond are significant.
Practice - Drawing Stereoisomers
Practice - Drawing Stereoisomers
Example 10. Draw the following organic compounds.
Example 10. Draw the following organic compounds.
cis-but-2-ene and trans-but-2-ene
Example 11. Draw the following organic compounds.
Example 11. Draw the following organic compounds.
cis-pent-2-ene and trans-pent-2-ene
This lesson is now complete. Consolidate your learning by evaluating the self-check below and the assigned practice questions.
Self-Check
(This is a self-assessment of your learning – you do not need to submit this).Self-Check
How prepared am I to start my homework? Can I ...
name and draw alkenes and alkynes?
name and draw stereoisomers for an alkene?
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