Organic Chemistry Worksheet Intermolecular Forces & Boiling Point

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1 Name________________________ Intermolecular Forces, Solubility and Melting Points Introduction Intermolecular forces are important concepts in organic chemistry. We can use it to explain: solubility, extraction, thin-layer chromatography, paper chromatography, gas chromatography, and column chromatography, among other topics. In order to have a good understanding of IM forces, we need to have a grasp of dipole moments: Determine whether each compound below exhibits a molecular dipole moment. Molecule Lewis Structure Does the molecule have a dipole? CH4 NH3 H2O CO2 CCl4 CH2Br2 Recall that there are five types of IM forces: London dispersion forces, dipole-dipole, H-bonding, iondipole, and ion-ion. Indicate the strongest type of intermolecular force present in the following compounds: Compound Butane CH3CH2OH NaCl CH3OCH3 IM Force Compound CH4 CH3COCH3 NaCH3O octane IM Force 2 Which of the following pure compounds will exhibit hydrogen boding? Draw the hydrogen bonding that takes place between: Molecules Drawing of H-bonding OH (ethanol) NH2 propylamine Physical Properties and IM Forces: The strength of intermolecular forces determines the physical properties (i.e. melting point, boiling point, solubility, etc…) of a compound. Generally speaking a molecule that has stronger intermolecular forces will have higher / lower (circle one) melting and boiling points. • Rank the following compounds in decreasing boiling point. o CH3OCH3 o CH3CH2OH o CH3CH2NH2 • Rank the following compounds in decreasing boiling point o CH3CH2OH o CH3(CH2)6CH2OH Identify the higher BP compound below and explain your reasoning: Draw the bond-line structures of the compounds 3 CH3CH2CH2OCH3 CH3CH2CH2CH2CH3 CH3CH2CH2CH2OH CH3CH2CH2CH3 Draw the bond-line structures of the following compounds and arrange them in increasing BP: (How do you think branching influences BPs?) 4 Solubility and Intermolecular Forces In the dissolving process the solute is usually the smaller quantity and it is the substance that is being dissolved whereas the solvent is the larger quantity and the substance that does the dissolving. The dissolving process can be broken down into the following steps. Solute-Solute Interactions The attractive forces (intermolecular) that hold the solute molecules together must be overcome, breaking the molecules apart. This step requires energy and the potential energy increases here. Solvent-Solvent Interactions The attractive forces (intermolecular) that hold the solvent molecules together must be overcome, breaking the molecules apart thus making room for the solute molecules. This step requires energy and the potential energy increases here also. Solute-Solvent Interactions During this process, the molecules of the solute interact with the molecules of the solvent. The solvent particles surround the solute particles and form attractive forces between them. This step decreases the potential energy. In order to dissolve, the solvent molecules and the solute molecules must break apart from each other and this process raises the energy. The new interactions between the solvent and the solute must be strong enough to lower the energy back down. We can look at IM forces to compare the interactions before dissolving to the interactions after dissolving. If the net forces are stronger (or similar with regard to IM forces) after dissolving, then the solute will dissolve in the solvent. Let’s examine the following scenarios. Solute IM Forces Weak London Dispersion Strong (Polar or H-bonding) Strong (Polar or H-bonding) Solvent IM Forces Weak London Dispersion Strong (Polar or ionic) Weak London Dispersion Solvent-Solute IM Forces Weak London Dispersion Strong (dipole-dipole or H-bonding) Weak Solubility Soluble Soluble Insoluble 5 Now, let’s see if we can make a few predictions regarding solubility. Predict if the following solutes (benzophenone, malonic acid, and biphenyl) will be soluble, partially soluble, or insoluble in the solvents listed (water, methanol, and hexane). Compound Water H2O Methanol CH3OH Hexane Benzophenone O Malonic acid O O HO OH Biphenyl Part C Chemical Reactions Complete the reaction below and label the Lewis acid, Lewis base, nucleophile, and electrophile. Is the product or reactant more water soluble. Explain why. O + OH- OH Complete the reaction below and label the Lewis acid, Lewis base, nucleophile, and electrophile. Is the product or reactant more water soluble. Explain why. O H 2N + H+ O For each of the following pairs of solutes and solvent, predict whether the solute would be soluble or insoluble. After making your prediction, you can check your answer if you would like by looking up the compounds in the Merck Index. If the substance has a solubility greater than 40 mg/mL, you 6 may conclude that it is soluble. In addition, redraw the bond-line structures as partially condensed structures. Solubility Partially condensed structure A Malonic acid in water O O HO OH B Anthracene in water C Acetaminophen in ethanol (CH3CH2OH) HO O N H D Aspirin in water O O E OH O Succininic acid (CH3(CH2)4CH3) in hexane 7 O HO OH O F Ibuprofen in ether (CH3CH2-O-CH2CH3) HO O G 1-decanol in water HO H Caffeine in methylene chloride O N N N N O Predict whether the following pairs of liquids are miscible or immiscible. 8 Ethanol Methanol Hexane CH3CH2OH CH3OH CH3(CH2)4CH3 Benzene Methylene Chloride CH2Cl2 Toluene Isopropyl Alcohol OH Miscible or Immiscible? A Water and methanol B Hexane and benzene C Methylene chloride and cyclohexane D Water and toluene E Hexane and water F Ethanol and isopropyl alcohol Would you expect ibuprofen (see above for structure) to be soluble or insoluble in 1.0M NaOH(aq)? Briefly explain your answer and Draw the reaction between NaOH and ibuprofen. Thymol is only very slightly soluble in water but very soluble in 1.0M NaOH(aq). Explain. HO Although cannabinol and methanol are both alcohols, cannabinol is only very slightly soluble in methanol at room temperature. Explain why. 9 1. Consider the three compounds below and answer the questions that follow. H H C H H H C C H H N H H H H H H C C C H H H H C C H C N H C C H H H C N H H H Compound C Compound A Compound B Part 1: Build a model of each of the compounds and answer the questions in the table below. Compound “A” Bond-line Structure How many pi bonds ? How many unhybridized p-orbitals? How many sp-orbitals? How many sp2 -orbitals? Part 2: Answer the following questions: a. Which two compounds are constitutional isomers? Compound “B” Compound “C” 10 b. Which compound contains a nitrogen atom with trigonal pyramidal geometry? c. Identify the compound with the greatest number of sigma-bonds. d. Identify the compound with the fewest number of sigma-bonds. e. Which compound contains more than one pi-bond? f. Which compound contains an sp2- hybridized carbon? g. Which compound contains only sp3- hybridized atoms (in addition to hydrogen atoms)? h. Which compound do you predict will have the highest boiling point? Explain. Build the following molecules and answer questions: Condensed Formula Compound “A” Compound “B” CH3CH3 CH2CH2 Compound “C” CHCH Compound “D” Benzene Lewis Structure How many pi bonds ? How many unhybridized porbitals? How many sp Orbitals How many sp2orbitals? How many sp3 orbitals? Now, draw a 3-D representation of the molecules showing how the orbitals overlap to form bonds. 11 Drawing of CH3CH3 Drawing of CH2CH2 Drawing of CHCH Drawing of Benzene

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Aljon2017
School: Rice University

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1
Name________________________
Intermolecular Forces, Solubility and Melting Points

Introduction
Intermolecular forces are important concepts in organic chemistry. We can use it to explain: solubility,
extraction, thin-layer chromatography, paper chromatography, gas chromatography, and column
chromatography, among other topics.
In order to have a good understanding of IM forces, we need to have a grasp of dipole moments:
Determine whether each compound below exhibits a molecular dipole moment.
Molecule
Lewis Structure
Does the molecule
have a dipole?
CH4
None

NH3

Yes

H2O

Yes

CO2

None

CCl4

None

CH2Br2

Yes

Recall that there are five types of IM forces: London dispersion forces, dipole-dipole, H-bonding, iondipole, and ion-ion. Indicate the strongest type of intermolecular force present in the following
compounds:
Compound
Butane

IM Force
Dipole-dipole

Compound
CH4

CH3CH2OH
NaCl
CH3OCH3

Dipole-dipole
Ion-ion
London dispersion
forces

CH3COCH3
NaCH3O
octane

IM Force
London dispersion
forces
Dipole-dipole
Ion-ion
London dispersion
forces

2

Which of the following pure compounds will exhibit hydrogen boding?

Answer:
The following will exhibit hydrogen bonding:
a) CH3CH2OH
f) CH3NH2
h) NH3

Draw the hydrogen bonding that takes place between:
Molecules

Drawing of H-bonding
OH (ethanol)

NH2

propylamine

Physical Properties and IM Forces:
The strength of intermolecular forces determines the physical properties (i.e. melting point, boiling point,
solubility, etc…) of a compound. Generally speaking a molecule that has stronger intermolecular forces
will have higher / lower (circle one) melting and boiling points.


Rank the following compounds in decreasing boiling point.
o CH3OCH3
o CH3CH2OH
o CH3CH2NH2

3
Answer: Ranking from highest to lowest,
1. CH3CH2OH
2. CH3CH2NH2
3. CH3OCH3


Rank the following compounds in decreasing boiling point
o CH3CH2OH
o CH3(CH2)6CH2OH

Answer: Ranking from highest to lowest,
1. CH3(CH2)6CH2OH
2. CH3CH2OH

Identify the higher BP compound below and explain your reasoning:
Draw the bond-line structures of the compounds

Branching decreases the boiling point. As the
length of carbon chain increases, the surface
area of the compound will also increase. Van der
Waals dispersion force is proportional to the
surface area. So, the increase of surface area
increases the ability of individual molecules to
attract each other. Branching in molecules
decreases the surface area thereby decreasing
the attractive force between individual molecules.
As a result, the boiling point decreases.

As the number of carbon atoms increases or the
length of carbon-carbon chain increases, the
boili...

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