When learning about each specific type of organic compound, in addition to understanding their structure and chemical properties, writing isomers is also very important. For the quiz format, how to calculate isomers will help you a lot, save time in tests and exams.
Viewing: what is isomerism
Then the formula and how to calculate isomers How, please refer to the following article.
How to calculate isomers
I. Isomer:
1. What is an isomer?
Isomers are compounds that are different but have the same molecular formula.
Isomers of each other have different properties because they have different chemical structures.
Note: Isomers of each other have the same molecular mass, but substances with the same molecular mass may not be isomers of each other.
For example, , but they are not the same CTPT so they are not isomers.
2. Classification:
We have many ways of classifying isomers, which can be based on bond order, functional groups or position in space, etc.
How to calculate isomers
Structural isomers:
+ C-chain isomer: obtained when changing the bonding order of C atoms together (straight chain, branch chain, ring).
Example: Together with a formula C4H10 we have the isomers:
Butane 2 – methylpropane
+ Co-classification of functional groups:
|
Functional groups |
Type of substance |
|
– OH |
alcohol |
|
– O – |
Ete |
|
– GIVE |
Andehit |
|
– CO |
Ketones |
|
– COOH |
Acid |
For example: With the same formula C2H6O we can write 2 isomers with 2 different types of functional groups (alcohol and ether):
Ethyl alcohol Dimtyl ether
+ Position isomerization of functional groups or multiple bonds: the position of functional groups, substituents or multiple bonds on the C chain changes.
Example: The C4H8 isomer has an open chain, in which there is a double bond:
– The cis-trans isomer:
Example with butene – 2 – en
It is found that when the two substituents of the C atom carrying the double bond are different, geometric isomers will appear. If the substituents ( -CH3, -C2H5, -Cl, …) have larger molecular masses on the same side of the double bond, it will be the cis form, the other side will be the trans form.
3. Steps to write isomers:
– Calculate the number of bonds π and rings:
+ v =
– Based on the molecular formula, the number of bonds π+v to choose the right type of substance. Usually the problem will be to write the isomer of a particular compound.
– Write the main C circuit:
+ Open circuit: Unbranched, 1 branch, 2 branches, …
+ Loop circuit: branchless ring, branched hoof ring, ….
– Attach functional groups or smear bonds (if any) to the circuit. Then move to change position. Symmetry should be considered to avoid duplication.
– Fill in H to ensure the valency of the elements. For the test, there is no need.
II. How to calculate isomers:
1. Calculation of isomers: Alkanes
– Definition: Alkanes are saturated, open-chain hydrocarbons. In alkanes, there are only CC and CH single bonds.
– Master plan: CnH2n+2 (n ≥ 1).
– Alkanes only have carbon chain isomers and from C4 onwards, isomers appear.
Quick formula:
Apply: Calculate the number of alkane isomers C4H10:
Instead of writing
Butane 2 – methylpropane
We use the above formula for n = 4, isomer.
With C5H12, we have n = 5 alkane isomers that will be isomers.
2. How to calculate the number of isomers: Alkenes
Alkenes are unsaturated, open-chain hydrocarbons that contain a double bond in the molecule.
– Fatherland Front: CnH2n (n ≥ 2).
– Alkenes have C chain isomers, double bond position isomers and geometric isomers.
– Quick tips for calculating alkene isomers:
Considering 2C with double bond, each C will bond with 2 substituents (same or different).
Example with C4H8: Subtracting 2C with double bond will leave 2C and H substituent.
If the problem requires calculating isomers, the structure will be: 1+1+1=3 isomers. If isomerism is required (including geometric isomers) it will be 1+1+2=4 isomers.
Example with C5H10: Subtracting 2C with double bond will leave 3C and H substituent.
We have 5 structural isomers and 6 alkene isomers.
3. Calculation of isomers: Alkin
Alkynes are unsaturated, open-chain hydrocarbons that contain a triple bond in their molecules.
– Fatherland Front: CnH2n-2 (n ≥ 2).
– Alkynes have C chain isomers, triple bond isomers and no geometric isomers.
– Quick tips for calculating alkine isomers:
Considering 2C with triple bond, each C will bond with 1 substituent (same or different).
Example with C4H6: Subtracting 2C with triple bond will leave 2C and H as substituent.
We have 2 alkene isomers.
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Example with C5H8: Subtracting 2C with double bond will leave 3C and H substituent.
We have 3 alkene isomers.
4. How to calculate the number of isomers: Benzene and homologues
– Benzene homologues are aromatic hydrocarbons that contain a benzene ring.
– Fatherland Front: CnH2n-6 (n ≥ 6).
– Formula to calculate isomers:
Application: Calculate the number of isomers of aromatic hydrocarbons C7H8, C8H10, C9H12.
We have n = 7, substituting the formula we get (7-6)2 = 1
n = 8, we get (8-6)2 = 4
n = 9, substitute the formula (9-6)2 = 9 isomers.
5. Calculation of isomers: Alcohol
– Alcohols are organic compounds in the molecule containing -OH group directly attached to C no.
– Alcohol functional group: -OH.
– Structure of saturated, monofunctional, open-chain alcohol: CnH2n+1OH or CnH2n+2O (n≥1).
Alcohols have C chain isomers and position isomers of OH group.
– Formula to calculate isomers:
– Apply: Calculate the number of alcohol isomers, monofunctional, open-chain from C3 → C5
C3H7OH: 23-2 = 2 isomers.
Propane – 1 – ol Propane – 2 – ol
C4H9OH: 24-2 = 4 isomers.
C5H11OH: 25-2 = 8 isomers.
Note: When writing alcohol isomers, the OH group cannot attach to the unsaturated C and a C atom cannot attach 2 or 3 OH groups.
6. Calculation of isomers: Ete
An ether is an organic compound in which an oxygen atom is bonded to two alkyl groups.
– Structure of ethers, monofunctional, open circuit: CnH2n+2O (n≥2).
– Formula to calculate isomers:
– Application: With n = 3, we have the ether formula C3H8O, substituting the formula we get: the isomer is
For n = 4, the ether formula is C4H10O, we are isomers.
Note: Alcohols and ethers, monomers are isomers in terms of functional groups. When writing isomers, if the question does not mention alcohol or ether, both types must be counted.
7. How to calculate the number of isomers: Phenol
Phenols are organic compounds in which the –OH group is directly bonded to aromatic C.
– General treatment: CnH2n-6O (n 6)
Quick formula:
– Apply:
C7H8O: 37-6 = 3 isomers.
With the same molecular formula but an aromatic alcohol isomer (OH attached to C no).
8. Calculation of isomers: Andehite
– An aldehyde is an organic compound in a molecule that has a –CH = O group directly attached to a C atom or an H atom.
– Structure of saturated, monofunctional, open-chain aldehydes: CnH2nO (n ≥ 1). The molecule contains a double bond in the CHO functional group.
Quick formula:
– Apply: Calculate the number of the following aldehyde isomers: C3H6O, C4H8O.
C3H6O, C4H8O are formulas of saturated, monofunctional, open-chain aldehydes.
With C3H6O: isomer.
With C4H8O: isomer.
9. How to calculate the number of isomers: Ketone
– Ketones are organic compounds in molecules that have a C = O group directly bonded to two C atoms.
– The chemical composition of saturated, monofunctional, open-chain ketones: CnH2nO (n ≥ 3). The molecule contains a double bond at the CO functional group.
– Formula to calculate isomers:
– Apply: Calculate the number of ketone isomers C5H10O
From the formula we can calculate: isomer.
Note: The aldehyde and the ketone have the same molecular formula, so when the problem is only for CTPT without mentioning any type of compound, both must be calculated.
10. How to calculate the number of isomers: Acid
Carboxylic acids are organic compounds in which the –COOH group is directly bonded to a C atom or an H atom.
– Structure of saturated, monofunctional, open-chain acids: CnH2nO2 (n ≥ 1). The molecule contains a double bond at the COOH functional group.
– Formula to calculate isomers:
– Apply: Calculate the number of acid isomers with CTPT C4H8O2
According to the formula we have: 24-3 = 2 isomers.
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Summary:
Above is how to calculate isomers of organic compounds commonly encountered in the Chemistry 11 program. However, these formulas only apply to a few basic cases, and have many limiting conditions.
So, in addition to manipulating how to calculate isomers In the multiple-choice exercises, you also need to know the steps of writing isomers of different compounds
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