Chemistry Module 2- Lesson 8
Chemical lesson 8
Study guide - Notes – Questions
In this lesson we focus on carbon compounds alkenes and alkynes with
double and triple carbon bonds. Also we look at polymers.
The weekly program
Develop your own method by all means but this will get you going.
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Read the notes attached here. All questions and the final assessment
are based on what is in the notes.
Read the chapter sections in the reading section ( the text has much
greater detail than what you are expected to know) especially look at
diagrams and figures
At this stage not much will make sense but that is OK
With your text as reference and this study sheet go through the Power
point presentation. Make notes where needed.
MANDATORY watch all of the embedded video links (you will need
internet access to do this)
Optional - Listen to the audio file of a live lesson; be aware that there
will be long pauses with not much going on at certain points.
Refine your notes / mind maps on the key concepts outlined in these
weekly study sheets. Check your understanding
Now go through your homework questions and answer those
Your study resources
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Textbook
Audio files of the actual lessons
Weekly study sheet
Power Point slides
YouTube links and YouTube as a general resource
College forum site
Textbook reference Bettleheim Edition 9
Chemistry Module 2- Lesson 8
Chapter 12 Section 12.1 12.2 12.5 Chemical connections 12.c
Key Concepts to understand
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Alkenes definition
Alkene structures
Isoprenes
Terpenes
Terpenoids
polyenes
Alkynes
Polymers
Use the links in the power point presentation.
Bozeman science
http://www.bozemanscience.com/
Crash course chemistry – YouTube
Socratic.org
https://socratic.org/
Please view these links as many times as you like especially for the
information that at present you find difficult. There are many other chemistry
clips on YouTube to explore and view to your heart’s content
Chemistry Module 2- Lesson 8
Study Notes
Alkenes
Alkenes are hydrocarbons that contain a carbon-carbon double bond. The
number of hydrogen atoms in an alkene is double the number of carbon
atoms. For example, the molecular formula of ethene is C2H4, while for
propene it is C3H6.
Alkenes are named with the prefix of – ene The table below shows three
alkenes.
Structure of alkenes
alkene
molecular formula structural formula
ethene
molecular model
C2H4
propene C3H6
butene
C4H8
Alkenes are unsaturated, meaning they contain a double bond that precludes
the full number of hydrogen carbon bonds. Due to the presence of the double
Chemistry Module 2- Lesson 8
bond and the fact that molecules cannot rotate around it alkenes have cistrans isomers.
Remember that cis means all the molecules are on one side of the double
bond and trans means they are on opposite sides of the double bond.
Alkenes can also exist in a ring formation where they are called cycloalkenes.
Alkenes that have two double bonds are called dienes, three bonds trienes
and with many carbon double bonds are called polyenes.
Alkenes with more than one double bond have cis-trans isomers in increasing
number. The relationship is 2n where n equals the number of double bonds.
It is worthwhile studying the section 12c in the textbook to see how important
isomers can be. The example is outlined below.
Chemistry Module 2- Lesson 8
You often hear of the beneficial effects of beta carotene content in vegetables.
The beta carotene molecule is an example of a polyene. Beta carotene is the
precursor molecule for the formation of retinal and vitamin A – Retinol.
The importance of a molecule and its conformation (cis – trans) is clearly seen
in the case of Vitamin A and eyesight. Beta carotene is split into two
molecules to make Retinol Vitamin A or more accurately 11 trans retinol.
Vitamin A retinol is converted by enzymes in the liver to 11 cis retinol which
the blood stream transports to the capillaries in the eye. Further changes
make the molecule 11 cis retinal
This molecule has a protein molecule called opsin added which converts it
into Rhodopsin.
Rhodopsin absorbs light energy and changes the carbon structure to an 11
trans retinal. This initiates a nerve impulse via the optic nerve to the visual
cortex (sight)
The opsin molecule is removed and we are back with 11 trans retinol. So
you can see in this example each conformation of the molecule is used for
specific stages in the process.
Chemistry Module 2- Lesson 8
Chemistry Module 2- Lesson 8
Alkene examples
Ethene (Ethylene) is common chemical used to ripen fruit for commercial
markets.
Chemistry Module 2- Lesson 8
Isoprene
Isoprenes are a foundational organic structure in nature and are an alkene.
They have two double bonds as you can see below. Joined together in
multiples they form molecules called Terpenes.
Two isoprene molecules joined
Isoprene molecule
Chemistry Module 2- Lesson 8
Using your model kit create these molecules and see how you can join them
together as shown in the diagram.
Terpene example - Myrcene in Bay leaf essential oil
Chemistry Module 2- Lesson 8
Terpenes are derived biosynthetically from units of isoprene. The basic
molecular formula of terpenes are multiples of that, (C5H8)n where n is the
number of linked isoprene units. This is called the isoprene rule or the C5 rule.
The isoprene units may be linked together "head to tail" to form linear chains
or they may be arranged to form rings. One can consider the isoprene unit as
one of nature's common building blocks.
As chains of isoprene units are built up, the resulting terpenes are classified
sequentially by size as hemiterpenes, monoterpenes, sesquiterpenes,
diterpenes, sesterterpenes, triterpenes, and tetraterpenes. Essentially, they
are all synthesised by the enzyme terpene synthase.
Terpenes and terpenoids are the primary constituents of the essential oils of
many types of plants and flowers. Essential oils are used widely as natural
flavour additives for food, as fragrances in perfumery, and in traditional and
alternative medicines such as aromatherapy. Synthetic variations and
derivatives of natural terpenes and terpenoids also greatly expand the variety
of aromas used in perfumery and flavours used in food additives. Vitamin A is
a terpene.
Limonene molecule
Chemistry Module 2- Lesson 8
Terpenes are also released by trees more actively in warmer weather, acting
as a natural form of cloud seeding. The clouds reflect sunlight, allowing the
forest to regulate its temperature. Australian eucalypt forests emit large
amounts of terpenes.
The aroma and flavour of hops, highly desirable in some beers, comes from
terpenes. The picture below depicts the most common terpenes in hops.
Terpenoids
The difference between terpenes and terpenoids is that terpenes are
hydrocarbons, whereas terpenoids contain additional functional groups.
Terpenes when modified chemically, such as by oxidation or rearrangement
of the carbon skeleton, are generally referred to as terpenoids. Some authors
will use the term terpene to include all terpenoids. Terpenoids are also known
as isoprenoids.
Terpenoids can be thought of as modified terpenes, wherein methyl
groups have been moved or removed, or oxygen atoms added. Just like
Chemistry Module 2- Lesson 8
terpenes, the terpenoids can be classified according to the number of
isoprene units used:
Hemiterpenoids, 1 isoprene unit (5 carbons)
Monoterpenoids, 2 isoprene units (10C)
Sesquiterpenoids, 3 isoprene units (15C)
Diterpenoids, 4 isoprene units (20C) (e.g. ginkgolides)
Sesterterpenoids, 5 isoprene units (25C)
Triterpenoids, 6 isoprene units (30C) (e.g. sterols)
Tetraterpenoids, 8 isoprene units (40C) (e.g. carotenoids)
Polyterpenoid with a larger number of isoprene units
Terpenoids can also be classified according to the number of cyclic structures
they contain.
Triterpenes & Sterols
Sterols are derivatives of triterpenes with 26 or more carbon atoms that have
undergone a characteristic type of rearrangement. Commonly it is from the
triterpenes squalene.
Sterols, also known as steroid alcohols, are a subgroup of the steroids and an
important class of organic molecules. They occur naturally in plants, animals,
and fungi, with the most familiar type of animal sterol being cholesterol.
Cholesterol is vital to animal cell membrane structure and function and a
precursor to fat-soluble vitamins and steroid hormones.
Chemistry Module 2- Lesson 8
Cholesterol is an important sterol in animals. This compound is an
intermediate in the synthesis of other steroids in plants, but is rarely, if ever,
accumulated in plants.
Alkynes
Alkynes are where the organic molecule has a triple carbon to carbon bond.
Their molecular formula is CnH2n-2
Acetylene
Most commonly used with oxyacetylene welding of metals. Widely used in
industry, it is sourced from thermal cracking of hydrocarbons.
Cycloalkynes are seldom encountered, and are not stable in small rings due
to angle strain. Cyclooctyne has been isolated, but is very reactive, and will
polymerize with itself quickly. Cyclononyne is the smallest stable cycloalkyne
Alkynes in nature and medicine
The first naturally occurring acetylenic compound, dehydromatricaria ester,
was isolated from an Artemisia species in 1826. In the nearly two centuries
that have followed, well over a thousand naturally occurring acetylenes have
been discovered and reported.
Chemistry Module 2- Lesson 8
Polyynes, a subset of this class of natural products, have been isolated from a
wide variety of plant species, cultures of higher fungi, bacteria, marine
sponges, and corals. Some examples are cicutoxin, oenanthotoxin, falcarinol
and carotatoxin.
Cicutoxin is a poisonous polyyne and alcohol found in various plants, such as
the highly toxic water hemlock (Cicuta species)
Chemistry Module 2- Lesson 8
Polymers
Polymer plastics - are very large molecules made from many smaller
molecules called monomers. Alkenes are able to act as monomers because
they contain a double bond. They can join end-to-end in a reaction called
addition polymerisation. The polymers they form are called addition polymers.
In general:
A lot of monomers
→
a polymer molecule
For example:
ethene → polyethene
propene → polypropene
chloroethene → polychloroethene (also called polyvinylchloride or PVC).
Ethene (Ethylene) is widely used for manufacturing polymers. Ethene is
created by thermal cracking which means exposing ethane (alkane) to
temperatures 800-900 degree Celsius for fraction of a second which forms
Ethene an alkene (ethylene).
Other monomers can be used to form polymers as can be seen below.
Chemistry Module 2- Lesson 8
Chemistry Module 2- Lesson 8
Our friend the Isoprene molecule can also be used to create polymers as well
as can be seen above. Natural rubber was used for many years until new
methods and techniques in organic chemistry superseded this source.
Nature use polymers in many organism structures such as cellulose and
proteins.
Chemistry Module 2- Lesson 8
Lesson 8 Homework
Q1 Name a sterol molecule commonly found in animals but not in plants. Also
add what other compounds and structure this sterol helps form
Q2 What is the major type of isomer found in alkenes and explain how to
distinguish them
Q3 Explain what a polyene is and give an example molecule
Q4 Name four man made polymers
Q5 What is the difference between terpenes and terpenoids?
Q6 Name an important body process that uses the difference between cis and
trans in a molecule conformation
Q7 The isoprene molecule, why is it so important?
Q8 Define what an alkene and an alkyne are
Q9 What is the basic unit of a polymer and provide an illustration showing
how they join together. The illustration can be an image from the internet.
Q10 Name three polymers you would find in nature
Chemistry Module 2- Lesson 8