Lecture 2: Discussion on Hydrocarbons Continued Outcome of the One-Minute Paper

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Lecture 2: Discussion on Hydrocarbons
Continued
Outcome of the One-Minute Paper
 Needs Clarification
Condensed vs. Expanded vs. Molecular Formula
 Naming Cycloalkanes and Alkanes with Substituents
 Lewis-dot structures and assigning formal charges
 Molecular Polarity
 Pace …Rushed through examples at the end
 Went Well
 Naming Alkanes
 Group Activities … Retained concepts better
 Guided inquiry questions through the lecture

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Lecture 2: Discussion on Hydrocarbons
Continued
 Today’s Class Agenda
 Revisit Condensed vs. Structural vs. Molecular Formula
 Revisit naming alkanes with substituents and
cycloalkanes with substituents
 Properties of alkanes
 Reactions involving alkanes
 Combustion
 Halogenation
 Revisit Lewis-dot structures and Resonance (pages
148-152 and 157-166)
We will also do Group Activities, Finish off the ALE 1 and
check into lab drawers
2
Revisiting Drawing Structural Formulas for
Alkanes
 Expanded formula: carbons connected in a row …
bonds between all atoms shown
 Condensed formula: carbons connected in a row …
bonds of atoms attached to carbon shown as a group,
with subscripts showing number of atoms bonded to
each carbon atom
 Molecular formula: carbon and atoms attached to it are
shown as a group, with subscripts indicating the
number of each atom present
3
Structural Formulas for Alkanes
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
TABLE 11.3
Line-bond structure
4
Conformations
The groups attached to a C-C single bond
 Rotate around the bond,
 Give different relative arrangements called
conformations.
5
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
Revisiting Naming Alkanes With Substituents
Study the names of the following alkanes and cycloalkanes and then
answer the following questions (work in groups to determine the rules):
1.
2.
How are the names of the cycloalkanes with substituents determined?
What about alkanes with substituents?
Cl
1-chloro-3-ethylcyclohexane
1-ethyl-3-methylcyclohexane
methylcyclopropane
Cl
Br
Cl
Cl
2-bromo--chlorobutane
3-chloro-2-methylpentane
Cl
3,5-dichloro-3-methylheptane
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Guide To Naming Alkanes
So, to name alkanes, we do the following
1. We find the longest continues chain and write the name
2.
3.
4.
5.
6.
of that alkane
We number chain from end nearest substituent.
We locate substituents and name them in alphabetical
order
If cylcoalkanes, we use the prefix “cyclo” before the
alkane name
For Cycloalkanes, numbering starts with the substituents
that gives the lowest numbers to the other substituents
(thus clockwise or counterclockwise)
A hyphen is used between numbers and names, and
commas to separate two or more numbers
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Pause: ALE 1
 Lets revisit problems 10-13 of ALE 1. Try it on your
own. You have 5 minutes to do all of the problems.
8
Learning Check
 Write three isomers of C5H12 and name each one (use line-
bond formulas
Isomer #
1
Isomer Formula
Name
2
3
 Write three isomers of C5H10 and name each one
Isomer 1 (and name):
Isomer 2 (name):
Isomer 3 (name):
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Properties of Alkanes: A Learning Check
For the following pairs of hydrocarbon, which one has the
higher boiling point? Why?
1. butane or octane
2. hexane or 2,3-dimethylbutane
Properties of Alkanes
 Nonpolar, flammable in air, less dense than water
 1-4 carbons, gas at room temperature; 5-17 carbons,
liquid at RT; > 18 carbons, waxy solids at room temp.
 Effect of boiling point
> increases with increased # of carbons
> Decreases for branched alkanes
> Increases for cycloalkanes
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Classification of Carbon Atoms
The classification of carbon atoms indicates the
number of carbon atoms attached.
 A primary carbon (1°) bonds to one carbon atom.
 A secondary carbon (2°) bonds to two carbon atoms.
 A tertiary carbon (3°) bonds to three carbon atoms.
CH3
|
CH3 —CH2—CH2—CH3
CH3—CH—CH3
primary secondary
tertiary
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Reactions of Alkanes: Combustion
Alkanes are flammable in air and react with oxygen. Study the
following reactions and see if you can figure out what is
happening.
1. In each reaction, what products are formed?
2. Are the differences between the reactions?
C5H12 + 8O2

5CO2 + 6H2O + Energy
C3H8 + 5O2

3CO2 + 4H2O + Energy
C7H16 + 11O2 
7CO2 + 8H2O + Energy
What conclusions can we make about the reaction of
alkanes with oxygen?
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Learning Check
1.
Write the combustion reaction for ethane and balance.
2.
In the cells of our body, energy is produced by the
combustion of glucose (C6H12O6). There are series of
reactions involved, but we can write the overall combustion
of glucose in our cells. Write the combustion reaction for
glucose and balance.
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Halogenation of Alkanes (Substitution
Reaction)
Lets review the following reactions between alkanes and halogens.
Think about what is happening and propose explanation? (for each
reaction to occur, a light or heat is required)
CH3—CH3 + Cl2
CH3-CH2-CH3 + Br2
 CH3—CH2 —Cl
 CH3—CH2—CH2— Br
Br
H3C
CH2
CH3
+ Br2
H3C
C
H
CH3
1. How many Hs were present in the alkanes before reaction happened?
2. How many are present after the reaction happened?
3. What does this tell us about the process of halogenation?
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Summary: Halogenation of Alkanes
When alkanes react with halogens,
 The reaction is called substitution, one or more H atoms
are replaced with a halogen, usually Cl or Br.
 Light or heat is required.
 A mixture of halogenated products result, but we write the
equation with the monosubstituted product.
Example: ethane and chloride
light
CH3—CH3 + Cl2
CH3—CH2 —Cl
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Revisiting Covalent Bonds for Elements in
Organic Compounds
Fill out the following table for each element?
Element
Group #
Valence
Electrons
# of electrons
need to achieve
octet
Covalent
bond it
can form
H
C
N
O, S
F, Cl, Br, I
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Revisiting Lewis-Dot Structures


Lewis-Dot structures give us structural formulas which
we can get arrangement of atoms, bond order, and formal
charges as well as resonance
Rules For Lewis Dots
1.
2.
3.
4.
5.
6.
Structure has to agree with formula
All atoms have to be attached (molecular skeleton)
Structures with maximum octets are preferred
Correct # of electrons must be present
More electron negative atoms are accommodated first
Minimum charge separation preferred
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Lewis-Dot Structures
 Central Atom: guess central atom. Some suggestions:
 Never H
 Most single dots
 Least Electronegative
 Least Numerous
Let’s illustrate this on the board using the following examples:
CH4, NH3, CO2
 Formula for the # of possible bonds and possible charges:
# of bonds = ½ [(sum of valence e- if full) – (sum of val. e-  charge)]
# of charges = # val. electrons - [# bonds + 2(unshared pairs)]
Lets use CO2 as an example to illustrate this
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Examples of Lewis Dot Structures
 These are done on board






CH4
NH3
CO2
CO
CO32NO-
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Revisiting Molecular Polarity and Shape
 Know that we know how to figure out bonds between
atoms, how does molecular shapes impact molecular
polarity?
Predict shape of molecule from Lewis-dot structures
 Determine if there is polar bonds
 Do dipoles cancel out as a result of the molecule’s
shape? If no, than molecule is polar. If dipoles cancel,
then molecule is nonpolar
 The presence of lone pairs in central atom makes
molecules polar

Redo Problems 1-4 of ALE 1
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Resonance Structures (page 152)
 Resonance Structures: Two or more equivalent
structures (actual is intermediate) for the same
molecule
Pushing electrons away from one atom and towards
another is a movement of the charge
 Relative position of all atoms stays unchanged
 Equivalent resonance structures contribute equally to
resonance intermediate
 Arrow connecting resonance structures is doubleheaded ()

Example: Draw resonance structures of CO32O
O
C
C
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O
O
O
O
Revisiting Functional Groups
TABLE 11.9
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
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