Structural Formulas and Isomers

Isomers

Multiple Choice

Compounds that are composed of the same number and type of atoms but have them arranged in different ways are ___________.

• isomers

• isotopes

• polymers

• alkanes

Multiple Choice

There are two main forms of isomerism, which are?

• Structural isomerism and Stereoisomerism

• Stereoisomerism and Physical isomerism

• Chemical isomerism and Stereoisomerism

• Structural isomers and Geometric Isomers

Multiple Choice

In a stereoisomer, ________ means "on the same side" and ______ means "across from."

• cis; trans

• cis; L

• D; L

• trans; cis

Multiple Choice

Which of the following is true about structural isomers?

• Have the same molecular formula
• Have different physical and chemical properties.
• Have the same number of elements
• all of the above

Multiple Choice

A pair of molecules that exist in two forms that are mirror images of one another but cannot be superimposed.

• Structural isomers

• Sterioisomers

• Geometric isomers

• Enantiomers

Multiple Choice

Given the structural formulas:

Which two formulas represent compounds that are isomers of each other?

• A and B

• A and C

• B and D

• C and D

Multiple Choice

Given a formula representing a compound:Which formula represents an isomer of this compound?

Multiple Choice

Which formulas represent compounds that are isomers of each other?

What is isomerism?

• Isomers are molecules with the same molecular formula (i.e. the same number and type of atoms) but in which the atoms are arranged in a different way.

Two main types of isomerism

• Structural isomers have different structural formulae. Three types of structural isomerism are chain isomerism, positional isomerism and functional group isomerism.

• Stereoisomers have the same structural formula, but the 3D arrangement of atoms is different (enantiomers). Two types are cis–trans isomerism and optical isomerism.

Chain isomerism

• In chain isomers, the carbon chain is arranged differently.

• For example, hexane has several chain isomers, all with the molecular formula C₆H₁₄:

Positional isomerism

• In positional isomers, the functional group is attached to a different carbon atom.

• For example, chloropentane has several positional isomers, all with the molecular formula C₅H₁₁Cl:

Positional isomerism

• Positional isomerism also exists in alkenes with four or more carbon atoms.

• For example, hexene has several positional isomers, all with the molecular formula C₆H₁₂:

Functional group isomerism

• Functional group isomers contain different functional groups and so are members of different homologous series.

• For example, both alcohols and ethers have the general formula CnH2n+2O so they may be functional group isomers:

Multiple Choice

• Chain isomers

• Functional group isomers

• Positional isomers

Multiple Choice

• Chain isomers

• Functional group isomers

• Positional isomers

Multiple Choice

• Chain isomers

• Functional group isomers

• Positional isomers

Multiple Choice

• Chain isomers

• Functional group isomers

• Positional isomers

Multiple Choice

• Chain isomers

• Functional group isomers

• Positional isomers

Cis–Trans isomerism

• Molecules can rotate freely around single C-C covalent bonds, but not around C=C double bonds.

• Isomers differ in the arrangement of the groups attached to the carbons in the double bonds.

• These isomers cannot be superimposed on each other because the arrangement of the methyl groups is different.

Cis–Trans isomerism

• If an alkyl group or atom other than hydrogen is attached to each carbon then the isomers can be named either cis (‘on the same side’) or trans (‘on the opposite side’).

Limitations of cis–trans isomerism

• For example, is it possible to identify which of these halogenoalkanes is the cis isomer and which is the trans isomer?

• Instead, a different system is used for these type of molecules: E–Z notation.

  
  

Limitations of cis–trans isomerism

• In more complex organic compounds, in which multiple hydrogens have been substituted by different groups, isomers cannot be defined using the cis–trans notation.

E–Z isomerism

• The E–Z notation is used to identify stereoisomers that cannot be called cis or trans.

• Isomers are identified as either E or Z depending on what ‘priority’ is given to the groups attached to the carbon atoms in the double bond. The priority of these groups is determined by a complex series of rules.

• E represents the German word ‘entgegen’, and corresponds to trans isomers. The highest priority groups are on the opposite side of the double bond.

• Z represents the German word ‘zusammen’, and corresponds to cis isomers. The highest priority groups are on the same side of the double bond

Optical isomerism

• A molecule can exist as two isomers that are non-superimposable, mirror images of each other, just like a left hand and right hand.

• Optical isomers have the same physical properties, but they rotate polarized light in opposite directions.

Multiple Choice

Stereoisomers share the same structural formula.

• True

• False

Multiple Choice

Molecules cannot rotate about either single nor double covalent bonds.

• True

• False

Multiple Choice

Trans isomers mean the groups are at the same side of the double bond.

• True

• False

Multiple Choice

Trans isomers correspond to E isomers.

• True

• False

Multiple Choice

The prefixes cis- and trans- work for all types of stereoisomers.

• True

• False

Open Ended

How do you feel about this lesson? What are the topics that make you confuse at this moment?