Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

2. Molecular Representations

Intermolecular Forces

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Now that we understand how bonds keep atoms together, the next question is, what force keeps molecules together? Do they bond to each other as well? Let’s take a look. 

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concept

How IMFs are related to melting and boiling points.

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*Correction: The bp (boiling point) of "Ethanol" is 173˚ F or 78˚ C

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All these molecules have similar molecular weights; however they have vastly different boiling points. Why is that?

 

Intermolecular forces (IMF’s) keep molecules stuck together. Questions about boiling / melting points of molecules are always referring to the strength of IMF’s between molecules. 

Strength of IMF's

  1. Hydrogen bonding

Hydrogens attached to small, highly electronegative atoms can hydrogen bond. Namely N, O and F. 

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concept

How hydrogen bonding works.

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  1. Dipole-dipole

We like to call this the net dipole force because it’s only present when the molecule has a net dipole. 

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concept

How dipole-dipole forces work.

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Van der Waals Forces

Also known as London Dispersion Forces or Dipole-Induced Dipole Forces

All molecules possess Van der Waals Forces. They increase with the following properties:             


Size (the heavier, the stronger the force)

Shape:Ring > Straight Chains > Branched Chains

Content

This picture is trying to illustrate how rings have greater surface area for interactions than straight or branched chains. 

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concept

How Van der Waals forces work.

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Practice Questions: Now that we have learned about factors that influence the strength of IMF's let's apply them!

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Problem

PRACTICE:Which of the following pairs of molecules would have the highest boiling point?

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Problem

PRACTICE:Which of the following pairs of molecules would have the highest boiling point?

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Problem

PRACTICE:Which of the following pairs of molecules would have the highest boiling point?

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Problem

PRACTICE:Which of the following pairs of molecules would have the highest boiling point?