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The 14 aliphatic non-cyclic (open chain) alcohol and ether constitutional structural isomers of molecular formula C₅H₁₂O

Introduction to isomerism for molecular formula C₅H₁₂O  (see also summary diagram)

Percent mass composition of C₅H₁₂O based on the relative atomic masses

C= 12.01  H = 1.01  O = 16.00  and  M_r(C₅H₁₂O) = 88.17

Element composition of C₅H₁₂O by mass: 68.11% carbon  * 13.74% hydrogen  *  18.15% oxygen

Empirical formula  = molecular formula of C₅H₁₂O

Structural isomerism includes (a) carbon chain variation (usually need a minimum of 4 C atoms), (b) change in position of a substituent or functional group and (c) functional group isomerism where the atoms have a different configuration, usually with significant differences in chemical and physical properties.

All three are applicable for isomers of C₅H₁₂O

(a) carbon chain variation and variation of C-O-C chains in ethers rather than C-C-C

(b) variation of position of hydroxy/alcohol (OH) and methoxy (OCH₃) groups in the carbon chain.

(c) functional group isomerism - alcohol C-O-H group and ether C-O-C linkage

Stereoisomerism is where molecules have the same basic constitutional structural formula, but isomers differ in the 2D/3D arrangement of the atoms.

E/Z stereoisomerism was called 'geometrical isomerism' e.g. cis and trans isomers of alkenes or disubstituted cyclic alkanes where there are 3D spatial variations that are not mirror images and not super imposable.

E/Z geometrical isomerism not possible with isomers of C₅H₁₂O

R/S stereoisomerism was called 'optical isomerism', the pairs of isomers are called enantiomers which are 3D non-superimposable mirror image forms of the molecule. The molecule must have a chiral centre (a stereocentre), that is an asymmetric carbon atom with four different atoms/groups attached to it.

4 of the constitutional isomers can exhibit R/S optical isomerism.

Note

There are a total of 14 constitutional isomers of molecular formula C₅H₁₂O.

4 of the constitutional can exhibit R/S optical isomerism - examples of stereoisomerism, pairs of enantiomers (3 alcohols and 1 ether).

Therefore, there are a total of 18 distinct isomers of molecular formula C₅H₁₂O

In terms of constitutional isomers, there are 8 alcohols (3 exhibit R/S optical isomerism) and 6 ethers (1 exhibits R/S optical isomerism), so there are 8 stereoisomers.

C₅H₁₂O isomers are a good example of functional group isomerism (alcohol C-O-H linkage, ether C-O-C linkage)

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Details of the 14 constitutional isomers of molecular formula C₅H₁₂O

(including the 4 that can form pairs of R/S 'optical' isomers, enantiomers, and assignment rule quoted)

There are eight isomeric alcohols and six isomeric ethers of formula C₅H₁₂O

They are all open chain aliphatic compounds.

 

The 8 alcohols: molecules (1) to (8) Functional group C-OH (alcohol, suffix 'ol' in IUPAC names)

(1) pentan-1-ol, 1-pentanol, n-pentyl alcohol

, ,  ,

is a linear primary alcohol

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 5 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 3 : 2 : 2 : 2  2 : 1  (for equivalent protons)

 

 

(2) pentan-2-ol, 2-pentanol, , ,

(a linear secondary alcohol), that will exhibit optical (R/S) isomerism, the 2nd carbon atom is a chiral centre.

R (left) and S (right)

The R/S stereoisomers of pentan-2-ol  (enantiomers, optical isomers)

From the CIP assignment priority rule for R/S isomers: ₈O  >  ₆C₆C  >  ₆C₁H  >  ₁H

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 5 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 3 : 1 : (1) : 2 :  2 : 3  (for equivalent protons)

 

(3) pentan-3-ol, 3-pentanol , ,

 A linear secondary alcohol

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 3 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 6  (3+3) : 4 (2+2) : 1 : (1)  (for equivalent protons)

 

(4) 2-methylbutan-1-ol, 2-methyl-1-butanol,

  , ,

A branched primary alcohol that will exhibit optical (R/S) isomerism, the 2nd carbon atom C2 is a chiral centre.

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 5 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 3 : 2 : 1 : (3) :  2 : 1  (for equivalent protons)

R (left), S (right)

The R/S stereoisomers of 2-methylbutan-1-ol  (enantiomers, optical isomers)

From the CIP assignment priority rule for R/S isomers: ₆C₈O  >  ₆C₆C  >  ₆C₁H  >  ₁H

 

(5) 3-methylbutan-1-ol, 3-methyl-1-butanol, 

  , ,

 A branched primary alcohol

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of integrated peak areas: 6 (3+3) : 1 : 2 : 2 : 1  (for equivalent protons)

 

(6) 2-methylbutan-2-ol, 2-methyl-2-butanol, 

  , ,

A tertiary alcohol

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of integrated peak areas: 6 (3+3) : (1) : 2 : 3  (for equivalent protons)

 

(7) 3-methylbutan-2-ol, 3-methyl-2-butanol, ,

  , A branched secondary alcohol, will exhibit optical (R/S) isomerism

R (left), S (right)

The R/S stereoisomers of 3-methylbutan-2-ol  (enantiomers, optical isomers, C2 is chiral)

From the CIP assignment priority rule for R/S isomers: ₈O  >  ₆C₆C  >  ₆C₁H  >  ₁H

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 4 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 6 (3+3) : 1 : 1 : 1 : 3  (for equivalent protons)

 

(8) 2,2-dimethylpropan-1-ol, 2,2-dimethyl-1-propanol, ,

  , A branched primary alcohol

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of integrated peak areas: 9 (3x3) : 2 : 1  (for equivalent protons)

 

The ethers: molecules (9) to (14) Functional group: ether linkage C-O-C (prefix alkoxy in IUPAC name)

(9) 1-methoxybutane, butyl methyl ether, linear ether

, ,

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 5 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 3 : 2 : 2 : 2 : 3  (for equivalent protons)

 

(10) 2-methoxybutane, sec-butyl methyl ether , ,

This branched ether will exhibit optical (R/S) isomerism, C2 carbon atom is chiral

R (left), S (right)

The R/S stereoisomers of 2-methoxybutane  (enantiomers, optical isomers)

From the CIP assignment priority rule for R/S isomers: ₈O  >  ₆C₆C  >  ₆C₁H  >  ₁H

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 5 ¹³C (email if disagree?)

¹H NMR ratio of integrated peak areas: 3 : 1 : 3 : 2 : 3  (for equivalent protons)

 

(11) 1-ethoxypropane, ethyl propyl ether, linear ether, chain of C and O atoms

, ,

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 5 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 3 : 2 : 2 : 2 : 3  (for equivalent protons)

 

(12) 2-ethoxypropane, ethyl isopropyl ether, a branch ether

  , ,

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 3 : 2 : 1 : 6 (3+3)  (for equivalent protons)

 

(13) 1-methoxy-2-methylpropane , iso-butyl methyl ether,

  ,   A branched ether.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR proton ratio of integrated peak areas: 3 : 2 : 1 : 6 (3+3)  (for equivalent protons)

 

(14) 2-methoxy-2-methylpropane, tert-butyl methyl ether,  

, , , A branched ether.

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of integrated peak areas: 9 (3x3) : 3  (for equivalent protons)

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Extra information about the isomers of molecular formula C₅H₁₂O

Constitutional Isomers of C₅H₁₂O

The molecular formula C₅H₁₂O gives rise to 14 constitutional isomers: 8 alcohols and 6 ethers.

Four of these can exhibit R/S optical stereoisomerism (pairs of enantiomers, optical isomers), therefore there are a total of 18 distinct isomers.

Eight of these are alcohols (–OH functional group), six are ethers (R–O–R′ functional group), and three alcohols and one ether are optically active (have a chiral centre, an asymmetric carbon atom attached to four different atoms or groups).

IUPAC Name	Type	Structural Isomerism Subtype	Chiral
Pentan-1-ol	Alcohol	Functional group; positional	No
Pentan-2-ol	Alcohol	Functional group; positional	Yes
Pentan-3-ol	Alcohol	Functional group; positional	No
2-Methylbutan-1-ol	Alcohol	Functional group; chain; positional	Yes
3-Methylbutan-1-ol	Alcohol	Functional group; chain; positional	No
2-Methylbutan-2-ol	Alcohol	Functional group; chain; positional	No
3-Methylbutan-2-ol	Alcohol	Functional group; chain; positional	Yes
2,2-Dimethylpropan-1-ol	Alcohol	Functional group; chain, most branched	No
1-Methoxybutane	Ether	Functional group, positional	No
2-Methoxybutane	Ether	Functional group; positional	Yes
1-Methoxy-2-methylpropane	Ether	Functional group; chain; positional	No
2-Methoxy-2-methylpropane	Ether	Functional group; chain; positional	No
1-Ethoxypropane	Ether	Functional group; chain; positional	No
2-Ethoxypropane	Ether	Functional group; chain; positional	No

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Types of Isomerism Exhibited by isomers of molecular formula C₅H₁₂O

1. Structural (Constitutional) Isomerism

• Chain isomerism: differences in carbon-skeleton branching (e.g., pentan-1-ol vs. 2-methylbutan-1-ol).
• Positional isomerism: same skeleton but functional group in different positions (e.g., pentan-2-ol vs. pentan-3-ol).
• Functional-group isomerism: same formula but different functional groups (alcohols versus ethers).

2. Stereoisomerism

• R/S Optical isomerism: three alcohols (pentan-2-ol, pentan-3-ol, 2-methylbutan-1-ol, 3-methylbutan-2-ol and one ether (2-methoxybutane), each contain one chiral center and thus exist as enantiomeric pairs.

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Physical Properties: Trends and Comparisons of the isomers of molecular formula C₅H₁₂O

• Boiling points (see data table below):

• Alcohols (hydrogen-bond donors/acceptors) have much higher bps than their ether isomers.

• Within alcohols, increased branching lowers bpts (e.g., 2-methylbutan-1-ol bp < pentan-1-ol bpt).

• Solubility in water:

• Alcohols are appreciably more soluble due to hydrogen bonding.

• Ethers are less polar; limited water solubility and lower dipole moment.

• Density & viscosity:

• Linear alcohols tend to be more viscous and slightly denser than ethers of similar molar mass.

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A detailed table of the 14 isomers of C₅H₁₂O (8 alcohols and 6 ethers), including their IUPAC names, boiling points and dipole moments

IUPAC Names	Type	Boiling Point (°C)	~Dipole Moment (D)
1-Pentanol, pentan-1-ol	Alcohol	138	~1.6
2-Pentanol, pentan-2-ol	Alcohol	119	~1.8
3-Pentanol, pentan-3-ol	Alcohol	115	~1.8
2-Methyl-1-butanol, 2-methylbutan-1-ol	Alcohol	131	~1.6
3-Methyl-1-butanol, 3-methylbutan-1-ol	Alcohol	131	~1.6
2-Methyl-2-butanol, 2-methylbutan-2-ol	Alcohol	102	~1.7
3-Methyl-2-butanol, 3-methylbutan-2-ol	Alcohol	113	~1.7
2,2-dimethyl-1-propanol, 2,2-dimethylpropan-1-ol	Alcohol	113	~1.6
1-methoxybutane	Ether	90	~1.3
2-methoxybutane	Ether	92	~1.3
1-ethoxypropane	Ether	93	~1.3
2-ethoxypropane	Ether	94	~1.3
1-methoxy-2-methylpropane	Ether	95	~1.3
2-methoxy-2-methylpropane	Ether	96	~1.3

Notes:

• Melting and boiling points are approximate and can vary slightly depending on purity and measurement conditions.

• Dipole moments are estimated based on structural features and available data; ethers tend to have lower dipole moments than alcohols due to lack of hydrogen bonding.

• The alcohols tend to have higher boiling points than their isomeric ether due to hydrogen bonding via the OH group, absent in ethers.

• The alcohols also tend to have lower dipole moments than their isomeric ethers due to the more polar -OH group, absent in ethers.

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Chemical Reactivity and Relative Reactivity of the isomers of molecular formula C₅H₁₂O

• Alcohols:

• React with carboxylic acids to give useful esters - solvents, fragrances etc.

• Oxidation trend: primary → aldehyde → carboxylic acid; secondary → ketone; tertiary resist oxidation under mild conditions.

• Dehydration trend: tertiary > secondary > primary (tertiary dehydrate most readily via E1 mechanism).

• Substitution (SN1/SN2): tertiary > secondary for SN1; primary favour SN2.

• Ethers:

• Cannot react with carboxylic acids to give useful esters.

• Generally inert to base/weak acid; cleaved by strong acids (HI, HBr) via SN1 or SN2, depending on substitution.

• Resistant to oxidation and dehydration.

• Relative reactivity ranking (in typical substitution):
tertiary alcohol > secondary alcohol > primary alcohol >> ethers.

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Uses and Applications of the isomers of molecular formula C₅H₁₂O

• Pentanols: solvents, flavour/fragrance (ester) precursors, intermediates in ester synthesis.
• Branched alcohols: plasticizers, high-octane fuel additives (e.g., 2-methylbutan-2-ol derivatives).
• MATE (2-methoxy-2-methylpropane): octane booster in gasoline.
• Ethyl-propyl ethers: specialty solvents in organic synthesis and fragrances.

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Common Student Misconceptions involving the isomers of C₅H₁₂O

• “Only straight-chain alcohols exist” (overlooking branched isomers).
• Confusing ether isomer count with alcohols; forgetting functional-group isomers.
• Failing to recognize chiral centres in secondary alcohols on non-symmetrical chains.
• Misnumbering substituents due to chain-end symmetry.

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Exam Revision Tips for questions involving the isomers of C₅H₁₂O

1. Draw systematically:

   • Start with the longest chain; enumerate OH positions; then add branches.

   • Partition the carbon count for ethers: (1+4) and (2+3).

2. Learn definitions and examples of chain, positional, functional-group, and optical isomerism.

3. Practice CIP rules for assigning R/S configurations to chiral alcohols.

4. Tabulate boiling-point trends vs. branching and functional group.

5. For mechanism questions (AP, IB HL):

   • Outline oxidation, substitution, dehydration mechanisms.

   • Connect tertiary alcohols to SN1/E1 pathways; primary to SN2/E2.

6. Board-specific pointers:

   • AQA/Edexcel/OCR/WJEC/CCEA: expect drawing all isomers and naming under timed conditions.

   • CIE/IB: focus on stereochemistry, optical activity calculations, specific rotation.

   • AP Honors: integrate mechanistic arrow-pushing with kinetics (rate laws) for SN1/SN2, acid-catalyzed conversions.

Practice multiple choice questions based on the isomers of C3H6O A set of randomized, technically rich multiple-choice questions based on the isomers of C5H12O (alcohols and ethers), tailored for A-level and pre-university chemistry courses including AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, and AP Chemistry. Each question includes: Structural, physical, or chemical differences Spectroscopic distinctions (¹H NMR, ¹³C NMR, IR) Esterification, solubility, boiling point trends Distractor analysis and exam tips You may have to sketch out some molecular structures to work out the answer. Jot down your responses and check out the answers  ANSWERS to the questions based on the isomers of C5H12O If you think there are any errors, please email me asap at chem55555@hotmail.com 1. Which isomer of C5H12O would show the fewest signals in a 13C NMR spectrum? pentan-1-ol 2-methylbutan-2-ol 1-methoxybutane 1-ethoxypropane 2. Which isomer is readily oxidised to an aldehyde? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 1-methoxybutane 3. Which isomer would show a broad O–H stretch in its IR spectrum around 3300 cm⁻¹? pentan-1-ol 1-methoxybutane 2-ethoxypropane 1-methoxy-2-methylpropane 4. Which isomer is most likely to form an ester with ethanoic acid? 2-methoxybutane 2-methylbutan-2-ol pentan-1-ol 1-ethoxypropane 5. Which isomer has the lowest boiling point? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 2-ethoxypropane 6. Which isomer would show a triplet and quartet in its ¹H NMR spectrum due to an ethyl group? pentan-1-ol pentan-2-ol 1-ethoxypropane 2-methylbutan-2-ol 7. Which isomer has a tertiary alcohol group? pentan-2-ol 3-methylbutan-2-ol 2-methylbutan-2-ol pentan-1-ol 8. Which isomer would show no O–H stretch in IR and no exchangeable proton in ¹H NMR? pentan-1-ol pentan-2-ol 2-ethoxypropane 3-methylbutan-1-ol 9. Which isomer would readily give a ketone on oxidation? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 1-methoxybutane 10. Which isomer would show the greatest number of non-equivalent hydrogen environments in 1H NMR? pentan-1-ol pentan-3-ol 3-methylbutan-2-ol 2,2-dimethylpropan-1-ol 11. Which isomer is most likely to undergo dehydration to form a stable alkene? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 3-methylbutan-1-ol 12. Which isomer would show a singlet in ¹H NMR for three equivalent methyl groups? 2,2-dimethylpropan-1-ol 3-methylbutan-2-ol pentan-2-ol 1-methoxybutane 13. Which isomer would not readily oxidise to an aldehyde or ketone? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 2,2-dimethylpropan-1-ol 14. Which isomer would show a strong C–O stretch in IR but no O–H stretch? pentan-1-ol pentan-2-ol 2-ethoxypropane 3-methylbutan-1-ol 15. Which isomer would show only four distinct 1H NMR signals? pentan-1-ol 1-ethoxypropane 1-methoxybutane 2-ethoxypropane 16. Which isomer has the most branched carbon skeleton? 2-methylbutan-2-ol pentan-1-ol pentan-2-ol 1-methoxybutane 17. Which isomer would show the fewest peaks in 1H NMR due to symmetry? 2-methylbutan-2-ol pentan-1-ol pentan-2-ol 1-methoxybutane 18. Which isomer would show the highest boiling point? pentan-1-ol 2-ethoxypropane 2-methylbutan-2-ol 1-methoxybutane  ANSWERS to the questions based on the isomers of C5H12O

Learning objectives - questions to be answered about molecular formula C₅H₁₂O?

How many constitutional isomers are there of formula C5H12O?

How do you draw the skeletal structure of isomers of C5H12O?

How do you draw the structural formula of isomers of C5H12O?

Can you recognise the different functional groups in the isomers of C5H12O?

How do you name the isomers of C5H12O?

How do you work out the structure of the isomers of molecular formula C5H12O?

How do you draw the structural formula and skeletal formula of the isomers of molecular formula C5H12O?

How do you name the isomers of molecular formula C5H12O?

How many aliphatic structural isomers are there of molecular formula C5H12O?

How many aliphatic carbon chain isomers are there of molecular formula C5H12O?

How many positional isomers are there of molecular formula C5H12O?

How many E/Z (geometrical) isomers are there of molecular formula C5H12O?

How many R/S (optical) isomers (enantiomers) of molecular formula C5H12O?

Are there any functional group isomers with a molecular formula C5H12O?

Does C5H12O have any stereoisomers?

This page will answer these questions for molecular formulae C₅H₁₂O

ANSWERS to the Practice multiple choice questions based on the isomers of C3H6O If you think there are any errors, please email me asap at chem55555@hotmail.com 1. Which isomer of C5H12O would show the fewest signals in a 13C NMR spectrum? pentan-1-ol 5 13C δ 2-methylbutan-2-ol 4 13C δ 1-methoxybutane 5 13C δ 1-ethoxypropane 5 13C δ  Answer: B. 2-methylbutan-2-ol Explanation: More symmetrical; fewer unique carbon environments. Distractors: A: Linear alcohol → more distinct carbons. C/D: Ethers and branched alcohols → more environments. Tip: Count unique carbon types, not total atoms. 2. Which isomer is readily oxidised to an aldehyde? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 1-methoxybutane  Answer: A. pentan-1-ol Explanation: B & C oxidised to ketones, D not readily oxidised Misconception: Mixing up class of alcohol with oxidation product Oxidation: primary → aldehyde → carboxylic acid; secondary → ketone; tertiary resist oxidation under mild conditions. 3. Which isomer would show a broad O–H stretch in its IR spectrum around 3300 cm⁻¹? pentan-1-ol 1-methoxybutane 2-ethoxypropane 1-methoxy-2-methylpropane  Answer: A. pentan-1-ol Explanation: Alcohols show broad O–H stretch; ethers do not. Tip: Ethers lack O–H bonds. 4. Which isomer is most likely to form an ester with ethanoic acid? 2-methoxybutane 2-methylbutan-2-ol pentan-1-ol 1-ethoxypropane  Answer: C. pentan-1-ol Explanation: Primary alcohols esterify easily. Distractors: A/D: Ethers don’t esterify. B: Tertiary alcohols tend to resist esterification. 5. Which isomer has the lowest boiling point? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 2-ethoxypropane  Answer: D. 2-ethoxypropane Explanation: Ethers lack hydrogen bonding → lower boiling points. Tip: Alcohols > ethers in boiling point due to H-bonding. 6. Which isomer would show a triplet and quartet in its ¹H NMR spectrum due to an ethyl group? pentan-1-ol pentan-2-ol 1-ethoxypropane 2-methylbutan-2-ol  Answer: C. 1-ethoxypropane Explanation: The 'isolated' ethyl group of the ethoxy group gives this characteristic splitting. Tip: Look for e.g. -O–CH2CH3 pattern, but NOT part of a longer alkyl chain, a bit subtle! 7. Which isomer has a tertiary alcohol group? pentan-2-ol 3-methylbutan-2-ol 2-methylbutan-2-ol pentan-1-ol  Answer: C. 2-methylbutan-2-ol Explanation: OH attached to tertiary carbon. Tip: Tertiary alcohols resist oxidation. 8. Which isomer would show no O–H stretch in IR and no exchangeable proton in ¹H NMR? pentan-1-ol pentan-2-ol 2-ethoxypropane 3-methylbutan-1-ol  Answer: C. 2-ethoxypropane Explanation: Ethers lack O–H groups. 9. Which isomer would readily give a ketone on oxidation? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 1-methoxybutane  Answer: B. pentan-2-ol Explanation: Oxidation: primary → aldehyde → carboxylic acid; secondary → ketone; tertiary resists oxidation under mild conditions. 10. Which isomer would show the greatest number of non-equivalent hydrogen environments in 1H NMR? pentan-1-ol 6 1H δ pentan-3-ol 4 1H δ 3-methylbutan-2-ol 5 1H δ 2,2-dimethylpropan-1-ol 3 1H δ  Answer: A. pentan-1-ol Explanation: Linear structure → more distinct environments. B is symmetrical C-C-C(OH)-C-C, C and D have equivalent methyl groups 11. Which isomer is most likely to undergo dehydration to form a stable alkene? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 3-methylbutan-1-ol  Answer: C. 2-methylbutan-2-ol Explanation: Tertiary alcohols dehydrate the most easily → more stable carbocation in mechanism. 12. Which isomer would show a singlet in ¹H NMR for three equivalent methyl groups? 2,2-dimethylpropan-1-ol 3-methylbutan-2-ol pentan-2-ol 1-methoxybutane  Answer: A. 2,2-dimethylpropan-1-ol Explanation: Three methyls attached to same carbon → singlet, no adjacent C-H proton. 13. Which isomer would not readily oxidise to an aldehyde or ketone? pentan-1-ol pentan-2-ol 2-methylbutan-2-ol 2,2-dimethylpropan-1-ol  Answer: C. 2-methylbutan-2-ol Explanation: tertiary alcohol Oxidation: primary → aldehyde → carboxylic acid; secondary → ketone; tertiary resist oxidation under mild conditions. 14. Which isomer would show a strong C–O stretch in IR but no O–H stretch? pentan-1-ol pentan-2-ol 2-ethoxypropane 3-methylbutan-1-ol  Answer: C. 2-ethoxypropane Explanation: Ethers show C–O stretch but lack O–H. 15. Which isomer would show only four distinct 1H NMR signals? pentan-1-ol 6 1H δ 1-ethoxypropane 5 1H δ 1-methoxybutane 5 1H δ 2-ethoxypropane 4 1H δ  Answer: D. 2-ethoxypropane Explanation: The most symmetrical molecule, two equivalent methyl groups 16. Which isomer has the most branched carbon skeleton? 2-methylbutan-2-ol pentan-1-ol pentan-2-ol 1-methoxybutane  Answer: A. 2-methylbutan-2-ol Explanation: Tertiary carbon with three methyl groups. 17. Which isomer would show the fewest peaks in 1H NMR due to symmetry? 2-methylbutan-2-ol 4 1H δ pentan-1-ol 6 1H δ pentan-2-ol 6 1H δ 1-methoxybutane 5 1H δ  Answer: A. 2-methylbutan-2-ol Explanation: High symmetry → fewer unique H environments, two equivalent methyl groups. 18. Which isomer would show the highest boiling point? pentan-1-ol 2-ethoxypropane 2-methylbutan-2-ol 1-methoxybutane  Answer: A. pentan-1-ol Explanation: Strong hydrogen bonding and linear structure, C is more compact, B & D lack OH group to facilitate the stronger intermolecular hydrogen bonding.  ANSWERS to the questions based on the isomers of C5H12O

Associated organic chemistry  links

 Advanced Level pre-university organic chemistry notes

 IR, mass and H-1 & C-13 NMR spectra of organic compounds

Index of sets of isomers for a given molecular formula

The molecular structure and naming of aliphatic ALCOHOLS including isomeric ethers

INDEX of ALL revision notes on the chemistry of ALCOHOLS (and mention of ethers)

For isomerism in organic chemistry, see also the notes

Isomerism: introduction, structural isomerism - chain, positional, functional group, tautomerism

Stereoisomerism: introduction, definition, priority rules, E/Z isomerism (cis/trans isomerism)

Stereoisomerism - R/S isomerism (optical isomerism) - definition - examples explained

 email doc brown - comments - query?

 This is a big chemistry website, please allow time to explore it

Index of advanced (pre-university) organic chemistry revision notes

 The chemistry of alkanes and the petrochemical industry

 The chemistry of alkenes

 The chemistry of organic halogen compounds

 The chemistry of alcohols

 The chemistry of aldehydes and ketones

 The chemistry of carboxylic acids and derivatives

 The chemistry of organo-nitrogen compounds

 The chemistry of aromatic compounds

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A summary chart of isomerism

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on isomerism (alcohol and ether isomers of C5H12O) are suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.

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