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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

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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

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Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Structural Formulas for Alkanes

TABLE 11.3

Line-bond structure

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Conformations

The groups attached to a C-C single bond Rotate around the bond, Give different relative arrangements called

conformations.

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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Revisiting Naming Alkanes With Substituents

1. How are the names of the cycloalkanes with substituents determined?

2. What about 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):

Cl1-chloro-3-ethylcyclohexane 1-ethyl-3-methylcyclohexane methylcyclopropane

2-bromo--chlorobutane

Br

Cl

3-chloro-2-methylpentane

Cl

3,5-dichloro-3-methylheptane

Cl Cl

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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 of that alkane

2. We number chain from end nearest substituent.3. We locate substituents and name them in alphabetical

order4. If cylcoalkanes, we use the prefix “cyclo” before the

alkane name5. For Cycloalkanes, numbering starts with the substituents

that gives the lowest numbers to the other substituents (thus clockwise or counterclockwise)

6. 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.

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Learning Check

Write three isomers of C5H12 and name each one (use line-bond formulas

Write three isomers of C5H10 and name each one

Isomer 1 (and name):

Isomer 2 (name):

Isomer 3 (name):

Isomer # Isomer Formula Name

1

2

3

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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 octane2. 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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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.

Learning Check

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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 —Cl

CH3-CH2-CH3 + Br2 CH3—CH2—CH2— Br

H3C CH2 CH3 + Br2 H3C CH

CH3

Br

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

Element Group # Valence Electrons

# of electrons need to achieve octet

Covalent bond it can form

H

C

N

O, S

F, Cl, Br, I

Fill out the following table for each element?

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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 Dots1. Structure has to agree with formula

2. All atoms have to be attached (molecular skeleton)

3. Structures with maximum octets are preferred

4. Correct # of electrons must be present

5. More electron negative atoms are accommodated first

6. 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 CO3

2-

NO-

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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 double-

headed ()

Example: Draw resonance structures of CO32-

C

O

O O

C

OO

O

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Revisiting Functional Groups

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

TABLE 11.9

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