Difficulty: Intermediate to Advanced | Prerequisites: Alkene reactions, stereochemistry (R/S, E/Z), cyclohexane conformations
This is one of the most stereochemistry-heavy sections in CHM 255. Epoxidation and ring opening require you to track the geometry of the starting alkene, the face of addition, and the regiochemistry and stereochemistry of the ring-opening step. Chair conformations tie it together by asking you to place substituents axial or equatorial. If cis/trans relationships on rings and R/S assignments feel uncertain, revise those before working through these problems.
Peracids (RCO3H) add an oxygen across a double bond to form an epoxide, preserving the alkene's cis/trans geometry. Acid-catalysed ring opening of the epoxide with water gives a diol with anti stereochemistry (the two –OH groups end up on opposite faces). Whether the diol is chiral or achiral depends on the symmetry of the starting alkene. Chair conformations of cyclohexane diols let you visualise the axial/equatorial arrangement and predict which conformer is more stable.
Epoxide (oxirane)
A three-membered ring containing one oxygen and two carbons. The ring strain (~60 kJ/mol) makes epoxides highly reactive electrophiles.
In simple terms, an epoxide is a tiny, strained ring that is desperate to open up.
Peracid (RCO3H)
An organic acid with an extra oxygen (a peroxy group). Common examples include mCPBA (meta-chloroperoxybenzoic acid). Peracids deliver an oxygen atom to an alkene in a concerted, stereospecific reaction.
Think of a peracid as an oxygen delivery truck that drops its cargo onto one face of the double bond.
Syn addition
Both new groups add to the same face of the double bond. Epoxidation itself is syn (the oxygen bridges the same face), but this term is most often used when discussing dihydroxylation with OsO4.
Anti addition
The two new groups end up on opposite faces. Acid-catalysed ring opening of an epoxide with water gives anti addition of two –OH groups, producing a trans diol.
Racemic mixture
A 50:50 mixture of two enantiomers. When an achiral starting material forms a chiral product, both enantiomers form in equal amounts.
In simple terms, you get equal amounts of the left-handed and right-handed versions.
Chair conformation
The most stable shape of a six-membered ring. Each position is either axial (pointing straight up or down) or equatorial (pointing roughly outward). Bulky substituents prefer the equatorial position.
Diaxial vs diequatorial
In a trans-1,2-disubstituted cyclohexane, one chair conformer places both groups equatorial (more stable) and the ring flip places both axial (less stable).
An alkene treated with a peracid (RCO3H, such as mCPBA) gives an epoxide. The reaction is concerted and stereospecific.
The oxygen is delivered to one face of the double bond in a single step.
The geometry of the alkene is preserved in the epoxide:
A (Z)-alkene (cis) gives a cis-epoxide.
An (E)-alkene (trans) gives a trans-epoxide.
If the alkene is symmetric with respect to the two faces, the peracid attacks both faces equally, giving a racemic mixture of epoxides.
For each alkene in Q8:
First assign (E) or (Z) using Cahn-Ingold-Prelog priority rules on the substituents attached to each carbon of the double bond.
Then draw the epoxide, keeping the relative positions of substituents exactly as they were in the alkene.
The peracid can attack from either face of a planar, symmetric alkene, giving two enantiomeric epoxides (a racemic pair).
Treating an epoxide with water under acid conditions (H2O, H+) opens the ring. Water acts as the nucleophile, attacking the more substituted carbon of the protonated epoxide (Markovnikov-like regiochemistry in unsymmetric cases).
The nucleophile (water) attacks from the back side, opposite to the departing oxygen.
This gives anti addition of the two –OH groups: they end up on opposite faces of what was the double bond.
The result is a trans-diol.
Whether the diol product is chiral depends on the symmetry of the molecule:
If the trans-diol has an internal plane of symmetry (a meso compound), it is achiral even though it has two stereocentres.
If no such plane exists, the diol is chiral, and the product is a racemic mixture of the two enantiomers.
To determine this for Q8: draw the diol with wedge-dash bonds, then look for a mirror plane through the molecule. If you find one, it is meso (achiral). If not, it is chiral (racemic).
Alkene + RCO3H → epoxide (syn addition of oxygen, geometry preserved)
Epoxide + H2O/H+ → trans-diol (anti ring opening)
The net result of both steps together is anti dihydroxylation of the alkene. This contrasts with OsO4, which gives syn dihydroxylation (cis-diol).
You start with cyclohexanol (–OH on a cyclohexane ring) and need to reach racemic trans-1,2-cyclohexanediol. This requires multiple steps because you need to install a second –OH group on the adjacent carbon with trans stereochemistry.
Step 1: Eliminate to form cyclohexene.
Treat cyclohexanol with a strong acid (e.g. H2SO4 or H3PO4, heat) to dehydrate the alcohol and form cyclohexene.
Step 2: Epoxidise the alkene.
Treat cyclohexene with a peracid (mCPBA or RCO3H) to give 1,2-epoxycyclohexane. The peracid attacks both faces of the symmetric alkene equally, giving a racemic epoxide.
Step 3: Open the epoxide with water under acid.
Treat the epoxide with H2O/H+ to open the ring via anti addition. Water attacks from the back side, placing the two –OH groups trans to each other.
The product is racemic trans-1,2-cyclohexanediol, a 50:50 mix of the (1R,2R) and (1S,2S) enantiomers.
OsO4 gives syn dihydroxylation, which would produce cis-1,2-cyclohexanediol (a meso compound). The question specifically asks for the trans diol, so the epoxide route is required.
For trans-1,2-cyclohexanediol, the two –OH groups must be on opposite sides of the ring (one up, one down, or equivalently, one wedge, one dash on adjacent carbons).
Chair A (diequatorial):
Both –OH groups sit in equatorial positions.
This is the more stable conformer because equatorial substituents avoid 1,3-diaxial interactions.
Chair B (diaxial):
Ring flip of Chair A places both –OH groups axial.
Less stable due to 1,3-diaxial interactions between each –OH and the axial hydrogens two carbons away.
Start by drawing a blank chair.
Place one –OH group on C-1. If it is equatorial and "up," the trans –OH on C-2 must be equatorial and "down" (trans on adjacent carbons means one up, one down in a chair when both are equatorial).
For the ring-flipped chair, every axial position becomes equatorial and vice versa. Both –OH groups that were equatorial are now axial.
The diequatorial conformer is always preferred. For –OH groups, the energy difference between axial and equatorial is about 2 x 0.7 = 1.4 kcal/mol favouring diequatorial, so the equilibrium lies strongly toward Chair A.
Students often confuse syn and anti addition. Epoxidation is syn (oxygen goes to one face), but the acid-catalysed ring opening is anti (nucleophile attacks from the opposite face of the departing oxygen). The two-step sequence (epoxidation then ring opening) gives overall anti dihydroxylation.
A common error is forgetting that a meso compound is achiral. If a diol has two stereocentres but also has an internal mirror plane, it is not chiral, and you should not call the product "racemic."
Students sometimes draw chair conformations with both –OH groups on the same side for a trans diol. In a trans-1,2-disubstituted cyclohexane, one substituent is up and the other is down. Both can still be equatorial (or both axial) because "up" and "down" are not the same as "axial" and "equatorial."
Forgetting to dehydrate the alcohol to the alkene first is a frequent mistake in Q9. You cannot epoxidise a C–C single bond.
⚠️ The epoxidation-then-ring-opening sequence is a classic exam question. You will almost certainly be asked to draw both the epoxide intermediate and the diol product with correct stereochemistry.
⚠️ Distinguishing syn dihydroxylation (OsO4 = cis-diol) from anti dihydroxylation (peracid then H2O/H+ = trans-diol) is a staple comparison question.
⚠️ Chair conformations of disubstituted cyclohexanes are tested repeatedly. Know how to draw both chairs and identify which is more stable.
⚠️ Deciding whether a diol is chiral or achiral (meso) requires you to look for a mirror plane, not just count stereocentres.
True or false: Epoxidation with mCPBA converts a (Z)-alkene into a trans-epoxide. (False. The cis geometry is preserved; you get a cis-epoxide.)
Fill in the blank: Acid-catalysed ring opening of an epoxide with water gives ______ addition. (Anti.)
True or false: A compound with two stereocentres is always chiral. (False. It could be a meso compound.)
Fill in the blank: In the more stable chair of trans-1,2-cyclohexanediol, both –OH groups are ______. (Equatorial.)
True or false: OsO4 gives a trans-diol. (False. OsO4 gives a cis-diol via syn dihydroxylation.)
Q: Starting from cyclohexanol, outline the steps to produce racemic trans-1,2-cyclohexanediol.
A: (1) Dehydrate with H2SO4/heat to give cyclohexene. (2) Epoxidise with mCPBA to give 1,2-epoxycyclohexane. (3) Open with H2O/H+ to give racemic trans-1,2-cyclohexanediol.
Q: An (E)-2-butene is treated with mCPBA, then H2O/H+. Is the diol product chiral or achiral?
A: The (E)-alkene gives a trans-epoxide. Anti ring opening gives a diol with both –OH groups anti. For 2,3-butanediol from (E)-2-butene via anti addition, you get the (2R,3R) and (2S,3S) enantiomers in equal amounts (racemic, chiral).
Q: Draw both chair conformations of trans-1,2-cyclohexanediol and circle the more stable one.
A: Chair A has both –OH groups equatorial (more stable). Chair B has both –OH groups axial (less stable). Circle Chair A.
Q: Why does the acid-catalysed ring opening of an epoxide give anti addition rather than syn?
A: The epoxide ring opens by back-side attack of the nucleophile (water) on the protonated epoxide. The oxygen leaves from one face, so the nucleophile must enter from the opposite face. This back-side requirement produces anti stereochemistry.
Q: Compare the products of treating cyclohexene with (a) OsO4 then NaHSO3, and (b) mCPBA then H2O/H+.
A: (a) OsO4 gives syn dihydroxylation: cis-1,2-cyclohexanediol (a meso compound, achiral). (b) mCPBA then H2O/H+ gives anti dihydroxylation: racemic trans-1,2-cyclohexanediol (chiral, two enantiomers).
Epoxidation connects back to alkene reactivity (electrophilic additions, covered earlier in the course). The anti ring opening parallels the stereochemistry of bromonium ion opening, which also proceeds anti. Chair conformations tie into the broader theme of conformational analysis from early in the semester, and they return when you study sugars (pyranose rings) later. The syn vs anti dihydroxylation comparison (OsO4 vs peracid/H2O) is one of the signature "compare and contrast" questions of the entire CHM 255 course.
Related Terms / Search Tags
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