Organic molecules
Nomenclature and properties of
organic substances
Learning objectives:
• Explain the differences between organic and inorganic
compounds.
• Explain the special characteristics of carbon that make
organic molecules unique.
• Explain and give examples of structural isomers.
• Describe the various functional groups and use these
functional groups to name and draw structural formulas
for various organic molecules.
• Relate the carbon chain length and various functional
groups present to the properties of the organic
molecules.
What are organic molecules and how
do they differ from inorganic
molecules?
• Organic molecules are molecules that contain
carbon. (With carbonates, CO, and CO2 being
exceptions.)
• Inorganic molecules are all other compounds.
• NOTE: Organic substances are called molecules, whereas,
inorganic are usually compounds (although they can be molecules).
What does this imply about the bonding that takes place between
atoms of organic substances?
Let’s compare organic and
inorganic substances!
• Organic
– Usually contain covalent
bonding within the
molecule
– Generally weak forces
between molecules
– Found usually as gases,
liquids, or low M.P. solids
– Usually flammable
– Low solubility in water
– Nonconductors
– Slow rate of chemical
reactions
• Inorganic
– Usually contain ionic
bonding within the
compound
– Generally strong forces
between formula units
– Found usually has high
M.P. solids
– Usually nonflammable
– High solubility in water
– Conductors
– Usually fast rate of
chemical reactions
Test Yourself
1) Which of the following are organic
molecules?
–
–
–
NaCl
Water (H2O)
C6H6 (Benzene)
–
–
–
–
A flammable substance
A substance with a low boiling point
A substance with strong intermolecular forces
A substance that is water soluble
- C8H18
- CaCO3
- CO
Why the difference?
• To explain the difference, we first must review some
basic theories from first level chemistry.
• Recall:
– Lewis (e- dot) Theory – A theory that explains nicely why atoms
bond in particular whole number ratios (Ex: NaCl, H2O).
– VSEPR Theory – A theory that explains nicely the observed
molecular shapes (Ex: Bent structure of a water molecule as
opposed to the predicted linear structure.)
– Molecular Orbital Theory – Explains the bonding between
atoms to give the unexpected shapes of certain molecules (Ex:
the tetrahedral arrangement of carbon atoms in a diamond.)
The difference is the carbon-carbon
bonding!!
• Carbon is unique in that it has 4 valence
electrons, 2s and 2p valence electrons
(therefore, you may think it will likely bond with
two other atoms through single bonds to pair up
the two unpaired electrons.) – However, we
observe this NOT to be the case for organic
molecules. We observe 4 single bonds and the
most common tetrahedral geometry.
• Why???
M.O. theory explains uniqueness of
carbon
• Recall: The 2s and 2p orbitals hybridize and form four sp3 hybrid
orbitals, intermediate in energy to the 2s and 2p orbitals. This
results in 4 unpaired electrons and is responsible for the observed
tetrahedral geometry.
• Carbon also likes to bond with itself (Recall the % ionic character of
zero due to no electronegativity difference between bonded atoms,
and therefore the purest non-polar covalent bond possible!). Some
of these carbon-carbon linkages can be enormously long (these are
macromolecules!)
– The covalent bonding is the primary reason for the weak forces between
molecules, and therefore for the lower melting and boiling points (easier
to break the molecules apart). This also explains the low solubility in
water (Recall: “Like dissolves Like” and water is a VERY polar
molecule.
Structural isomers?
• Due to the unique carbon-carbon bonding
and the various possibilities, structural
isomers (substances with the same
molecular formulas, but different bonding
arrangement of atoms) are more common
for organic compounds than inorganic
compounds.
Example of structural isomers
• Ethanol (Ethyl alcohol)
– Formula: C2H6O
– Liquid at room temperature
• Dimethyl ether
– Formula: C2H6O
• Gas at room temperature
Test Yourself!!!
• See the Learning Check 11.3 on Page 347
of the textbook.
How are all the organic substances
organized?
• They are characterized according to
characteristic structural features called functional
groups. Some examples are as follows:
– A carbon – carbon double bond (called alkenes)
– A carbon – carbon triple bond (called alkynes)
– Carbonyls ( a carbon atom double bonded to an
oxygen atom, of which there are various sub classes)
– For a complete listing, see the handout provided or
Table 11.2 on page 348 of the textbook.
What is Organic nomenclature?
• Nomenclature involves writing formulas for
and drawing structures for a substance.
For organic nomenclature, this involves
carbon molecules.
• When drawing structures, there are two
types:
– 1) Expanded structural formulas – show ALL
covalent bonds
– 2) Condensed structural formulas – Show
only specific bonds
What are hydrocarbons?
• The most common non-carbon atom
bonded to a carbon atom in an organic
molecule is the hydrogen atom. For this
reason, most organic molecules are also
called hydrocarbons.
Saturated vs. Unsaturated
hydrocarbons?
• Saturated hydrocarbons are hydrocarbons with
the maximum number of atoms bonded to the
carbons (These atoms are usually hydrogen
atoms or halogens). These are called alkanes
(contain NO carbon – carbon double or triple
bonds)
• Saturated hydrocarbons usually have a
molecular formula of CnH2n+2 (with n being the
number of carbon atoms in the molecule.)
Alkane nomenclature
• Use the following prefixes when naming alkanes (they vary
depending on the carbon chain length):
– 1 carbon:
meth
- 6 carbons: hex
– 2 carbons:
eth
- 7 carbons: hept
– 3 carbons:
prop
- 8 carbons: oct
– 4 carbons:
but
- 9 carbons: non
– 5 carbons:
pent
- 10 carbons: dec
– For alkanes, use the above prefix and the “ane” ending.
– For Ex: CH4 – methane or C3H8 – propane
– For a complete listing see table 11.4 on page 356.
Organic Nomenclature of branched
alkanes
• Branched alkanes are alkanes that do NOT form a long
continuous chain of carbon atoms. These consist of
alkyl groups. (These are alkanes missing a hydrogen
atom and serve as substituents of a longer parent chain.)
• Unbranched alkanes usually contain the n- prefix to
indicate an unbranched chain. The iso- prefix is used to
indicate a branched alkane with all carbon atoms on the
long continuous chain except one and that one is
branched next to end of the parent chain.
– Ex: n-butane
iso-butane
CH3
– CH3CH2CH2CH3
I
CH3CHCH3
How are alkyl groups named?
• Alkyl groups are named by identifying the alkane
root of the alkyl group, dropping the “ane”
ending and adding “yl”.
• For example:
• CH4 is methane
• CH3CH3 is ethane
-CH3 is methyl
-CH2CH3 is ethyl
• For a listing of some common alkyl groups, see
Table 11.5 on page 357.
Group
CH3–
Name
Methyl
C2H5–
Ethyl
CH3CH2CH2–
Propyl
(CH3)2CH–
Isopropyl
CH3CH2CH2CH2–
Butyl
(CH3)2CHCH2–
CH3CH2CH(CH3)–
Isobutyl
sec-Butyl
(CH3)3C–
t-butyl
Mini Quiz
•
Name the following alkyl groups:
CH3CH2CH2CH3CH2CH2CH2CH2CH3
I
CH3CHCH2CH3
I
CH3CCH3
I
Naming alkanes – using IUPAC
rules
• Step 1) Name the longest chain.
• Step 2) Number the longest chain. If there are attached
carbon groups, number so that they give the lowest
number to any attached carbon.
• Step 3) Locate and name any attached alkyl groups.
• Step 4) Combine the longest chain and the braches into
one name. Use numbers & dashes to indicate the
position of the branches on the longest chain (called the
parent chain).
• Step 5) If there are more than one of the same
branched group, use commas between the numbered
locations and use the prefixes di, tri, tetra, penta, etc…
to indicate the correct number of branched groups.
Examples:
•
Name the following alkanes:
1)
CH3CH2CH2CH2CHCH3
I
CH3
2)
3)
CH2CH3
I
CH3CH2CH2CHCH2CHCH3
I
CH2CH3
CH3
I
CH3CH2CH2CHCHCHCH3
I
I
CH
CH3
/ \
CH3 CH3
Now complete the self quiz by
linking to the site below –
complete quiz 1 at this time.
Click here for the self-quiz
http://www.chembio.uoguelph.ca/educmat/chm19104/organic_nomenclature_quizzes.htm
Drawing structural formulas
• To draw the structural formula for an
organic substance, simply reverse the
previous process.
• Examples: Draw structures for:
1) 2,2,4-trimethylpentane
2) 3-isopropylhexane
3) 3-ethyl-2,4-dimethylheptane
Properties of alkanes
• Alkanes are NOT water soluble (due to the
non-polar covalent bonding).
• As the length of the chain increases, the
boiling point increases (less likely to be a
gas at room temperature) and
consequently the density increases.
Test Yourself
• Which has a higher B.P., propane or
pentane?
• Is butane a liquid or gas at room
temperature?
• Is octane more or less dense that
pentane?
Cycloalkanes
• Cycloalkanes are alkanes containing rings of carbon
atoms.
• These are named using the same set of rules already
mentioned, only adding the prefix cyclo- before the
alkane name.
– If there is only one branched group, no number is needed to
indicate its location on the parent chain.
– If there are more than one branched groups, then number
beginning with the first group alphabetically and proceed around
the ring in a direction that gives the lowest numbering possible.
– See text or class notes for examples.
Special isomers
• In striaght chain alkanes, there is free rotation of
the carbon atoms. This is NOT possible in
cycloalkanes.
– Therefore, cycloalkanes have tighter C-C bonds and
are harder to break, making them less reactive!
– Due to restrictive nature of C-C bonds in
cycloalkanes, they can exist as geometric isomers
(Molecules with restricted C-C bonds that have
different 3-D arrangements of atoms at the restricted
C-C site.) These types of isomers are also common
for alkenes (C=C bonding).
Two types of geometric isomers
• Cis isomers
• Trans isomers
– Substituted groups
project in the same
direction from the rigid
C-C plane.
– Substituted groups
project in the opposite
direction from the rigid
C-C plane.
– Ex: cis-1,2dimethylcyclopentane
– Ex: trans-1,2dimethylcyclopentane
Test Yourself
• Draw the following:
– 1)
– 2)
– 3)
– 4)
– 5)
1,1-dimethylcyclobutane
trans-1,2-dimethylcycobutane
cis-1,3-dimethylcyclobutane
cis-1,2-dichlorocyclohexane
trans-1,3-dibromocyclopentane
Nomenclature of unsaturated hydrocarbons
(alkenes and alkynes and aromatics.)
• Alkenes – contain one or more C-C double bonds.
– When naming, change the ending of the name from “ane” to “ene” and
use the previously mentioned numbering system to indicate the location
of the double bond. The longest chain MUST include the double bond.
Have a general formula of CnH2n (for one double bond).
• Must consider cis & trans geometric isomers.
• Alkynes – contain one or more C-C triple bonds.
– When naming, change the ending of the name from “ane” to “yne” and
use the previously mentioned numbering system to indicate the location
of the triple bond. The longest chain MUST include the triple bond.
Have a general formula of CnH2n-2 (for one triple bond).
• Aromatics – includes cycloalkenes and the simplest aromatic,
benzene.
Try some alkene nomenclature
• Name the following:
– CH3-CH=CH-CH3
CH3-CH2
I
C=CH2
I
CH3-CH2-CH2
CH3-CH-CH=CH2
I
CH3
For more examples, see the handouts in class or try some
additional exercises from the book.
Properties of alkenes
• As the length of the alkene chain
increases, again we observe in increase in
the boiling point and the density of the
substance.
Aromatics
• Organic molecules
that have a benzene
derivative.
• Contains staggered
double bonds with
delocalized electrons.
Aromatic root names – memorize
these!!
• toluene
More aromatics
• analine
• phenol
Stereoisomers of aromatics
• Uses the following isomer nomenclature
when there are only 2 derivatives on a
benzene ring:
• 1 & 2 substituted are ortho (o-)
• 1 & 3 substituted are meta (m-)
• 1 & 4 substituted are para (p-)
Now complete the self quiz by
linking to the site below –
complete quiz 2 at this time.
Click here & take quiz 2
http://www.chembio.uoguelph.ca/educmat/chm19104/organic_nomenclature_quizzes.htm
Prepare for a TEST!!!!
End of Chapter 11 & 12 review
questions are a good review for
the test!!