Video Solution: We have seen in this chapter that, among isomeric alkanes, t...

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We have seen in this chapter that, among isomeric alkanes, the unbranched isomer is the least stable and has the highest boiling point; the most branched isomer is the most stable and has the lowest boiling point. Does this mean that one alkane boils lower than another because it is more stable? Explain. 

Video Transcript

Hey everyone. So in this question we're going to compare two different structures, we have a branch one and unbranched. Notice that the ones I drew are isomers of the molecular formula C4H10 to just remember that we're dealing with branched and unbranched, which one is going to have the higher stability? the branch one, okay? And then which was going to have the lower stability? the unbranched, okay? Now, despite that also their boiling points are opposites. Now, the branch one is going to have the lower boiling point, this branch is going to decrease the boiling point, so you can take a lower BP this one's going to have higher BP. Now, why is that? do you think it has to do with the molecule that's more stable always have the lower boiling point? Well, first let's talk about the boiling point, so the boiling points have to do with simply the shape of the molecule, so notice that our molecule on the left, our branch one is going to be more spherical than the other, it's going to have this smaller surface area, so we;re going ot say that branching has to do with the smaller surface area, okay? excuse me, the boiling point, okay?

Circle the compound with the highest boiling point:

a) CH4

b) CH3CH3

c) CH3Cl

d) CH3Br

e) CH3OH

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The following compounds have similar molecular weights. Which has the highest boiling point.

A)        CH3CH=O

B)        C2H5OH

C)        CH3OCH3

D)        CH3CH2CH3

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Which of the following molecules has the greater boiling point? 

1-hexylamine or cyclohexylamine

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In each pair of molecules below, select the one with the greater predicted boiling point. 

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For each of the following, choose the best answer; give the letter.

(a) Which has the lower heat of combustion: (i) (E)-3-hexene or (ii) 1-hexene?

 

(b) Which one has the higher heat of combustion: (i) (E)-cyclooctene or (ii) (Z)-cyclooctene?

 

(c) Which one has the lower boiling point: (i) chloroform or (ii) methylene chloride?

 

(d) Which one has the higher boiling point: (i) 1,1,1-trifluoroethane or (ii) hexafluoroethane?

 

(e) Which one is more dense: (i) 1-iodooctane or (ii) 1-bromooctane?

 

(f) Which one is more dense: (i) water or (ii) 1-iodooctane?

 

(g) Which one is thermodynamically more stable: (i) 3-methylcyclohexyl cation or (ii) 1-methylcyclohexyl cation?

 

(h) Which one can be the units of a first-order rate constant: (i) L mol−1 s−1 or (ii) s−1 ?

 

 

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Which compound has the lowest boiling point: 

(a) CH3CH2CH2CH2CH2CH3

(b) (CH3)2CHCH2CH2CH3

(c) (CH3)3CCH2CH3

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Most people are familiar with the idea that oil and water do not mix. Crude oil is a complex mixture of mostly very long chain alkanes. During the refining process, the long alkane chains of crude oil are broken into shorter chains using heat and catalysts, then purified by distillation at refineries on the basis of boiling point into fractions such as gasoline (boiling point fraction between 100°C and 400°C) or diesel fuel (boiling point fraction between 180°C and 360°C). During the Deepwater Horizon oil rig disaster a little over two years ago, between 17 and 39 million gallons of crude oil spilled into the Gulf of Mexico. Thousands of birds, turtles and sea mammals died because they were covered by oil. Well over 125 miles of coastline were affected. The major cause of the destruction is that the oil was not simply diluted by sea water, but rather, it floated on the surface in large slicks that were blown by prevailing winds until they reached a shore. What is worse, animals that need to breath air such as turtles and sea mammals (dolphins, whales, manatees) became trapped under the oil slick and had to surface into it to breathe, sealing their doom. Unsuspecting birds landed on the surface of the oil slick, becoming covered in sticky oil that prevented them from flying away.

As mentioned in the passage, when crude oil is refined, heat and catalysts break the long carbon chains into much shorter ones. The resulting mixture is distilled and fractions with specific boiling point ranges are collected together and sold according to their intended use such as jet fuel, gasoline, heating oil, etc. Based upon what you know about the boiling points of branched vs. straight chain alkanes, which statement do you think will NOT be true as you analyze what actual molecules are in each fraction isolated based on similar boiling points.

A. The fractions with higher boiling points will generally have larger alkane molecules (higher molecular weights).

B. In a given fraction, the branched molecules will on average have a higher molecular weight compared to the straight chain molecules.

C. In a given fraction, the branched molecules will on average have a lower molecular weight compared to the straight chain molecules. 

D. The fractions with lower boiling points will generally have smaller alkane molecules (lower molecular weights).

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Most people are familiar with the idea that oil and water do not mix. Crude oil is a complex mixture of mostly very long chain alkanes. During the refining process, the long alkane chains of crude oil are broken into shorter chains using heat and catalysts, then purified by distillation at refineries on the basis of boiling point into fractions such as gasoline (boiling point fraction between 100°C and 400°C) or diesel fuel (boiling point fraction between 180°C and 360°C). During the Deepwater Horizon oil rig disaster a little over two years ago, between 17 and 39 million gallons of crude oil spilled into the Gulf of Mexico. Thousands of birds, turtles and sea mammals died because they were covered by oil. Well over 125 miles of coastline were affected. The major cause of the destruction is that the oil was not simply diluted by sea water, but rather, it floated on the surface in large slicks that were blown by prevailing winds until they reached a shore. What is worse, animals that need to breath air such as turtles and sea mammals (dolphins, whales, manatees) became trapped under the oil slick and had to surface into it to breathe, sealing their doom. Unsuspecting birds landed on the surface of the oil slick, becoming covered in sticky oil that prevented them from flying away.

Why does oil float on the surface of sea water rather than sink to the bottom?

A. Oil is less dense than water primarily because crude oil is mostly -CH 2- chains while water is H2O and O has a higher atomic number than C.

B. Oil is less dense than water primarily because crude oil is mostly -CH 2- chains while water is H2O and O has a lower atomic number than C.

C. Oil is less dense than water primarily because water molecules cannot pack together as well as the crude oil molecules pack together.

D. All of the above.

The long chain alkanes of crude oil stick to each other largely through interactions that can be described as the following:

A. London dispersion forces.

B. Attraction between induced transient dipoles associated with temporary fluctuations in electron clouds around the molecules.

C. Hydrogen bonding

D. Both A and B

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Select the molecule with greater solubility in water. Provide a brief explanation.

Ethanol or Ethane

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Select the molecule with the highest boiling point.

Acetone (CH3COCH3) or Propanoic acid (CH3CH2COOH)

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Select the molecule with the highest boiling point.

Octane (CH3(CH2)6CH3) or Decane (CH3(CH2)8CH3)

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Describe all the intermolecular interactions in low molecular weight alcohol. Draw out your alcohols chemical structure and use pictures and bullet (brief) points to describe these interaction. 

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Predict the highest boiling point amine from those shown below 

(i) N(CH2CH3)3     (ii) HN(CH2CH2CH3)2          (iii) N(CH3)3          (iv) CH3(CH2)4CH2NH2

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Rank the following compounds from lowest to highest boiling point.

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Circle the highest melting point for each of the following series. 

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From the multiple options listed below (A through X), select the letter that correctly ranks the following compounds in order of decreasing boiling point.

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Rank the following compounds from lowest to highest boiling point.

diethyl ether      1,2-propanediol      1-butanol

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Answer the following questions based on the information provided about the following fatty acids below.

A) Which of the fatty acids (FA) above could be saturated or trans fats? 

B) Which of the FAs below has the lowest melting point?

C) Which of the FAs below has the highest melting point?

D) What kind of double bonds are in the FAs below?

 

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Which one of the following species forms the strongest ion-dipole attraction with 18-crown-6?

a) F -

b) K+

c) Cr2O72-

d) Br2

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Which of these has the highest boiling point?

a) H2NCH2CH2CH2CH2CH2CH2NH2

b) CH3NHCH2CH2CH2CH2NHCH3

c) (CH3)2NCH2CH2N(CH3)2

d) (CH3)2NCH2CH2CH2CH2NH2

e) The above are isomers and have the same boiling point

e) None of the above

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Which compound has the lower boiling point:

(a) CHCl 3 or (b) CH 2Cl 2? (Choose one)

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Please follow the steps below:

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Please follow the steps below:

A) Construct the Kekule or Lewis-Dash (including lone pairs) for the following molecule.

B) Use dipole arrows to denote any polar bonds.

C) Is the molecule polar?

D) What is the strongest intermolecular force present?

CH2CHCO2H

Circle one: Polar molecule? Yes or no

Strongest IMF: _________________

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Which compound has the higher boiling point:

a) CH3(CH2)3CH3

or

b) (CH3)4C

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Which compound has the lower boiling point?

a) CH3(CH2)2CH3

or

b) (CH3)2CHCH2CH3

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Which of these compounds would have the  highest boiling point?

a. I

b. II

c. III

d. IV

 

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