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Organic Chemistry I - Conjugated Dienes

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    Study Guide Organic Chemistry I – Conjugated Dienes 1. Preparation of Conjugated Dienes Conjugated dienes are a type of molecule that contains two double bonds in a row, with a single bond in between. They are made in reactions that are similar to those used to create regular alkenes. The two most common ways to create conjugated dienes are: 1. Dehydration of diols (alcohol molecules with two hydroxyl groups) — this removes water from the molecule. 2. Dehydrohalogenation of dihalides (molecules with two halogen atoms) — this removes a halogen atom. Whether the product is a conjugated diene or an isolated diene depends on how the original molecules were arranged. Conjugated vs. Isolated Dienes • Conjugated dienes : These have alternating single and double bonds, like in butadiene . They often form when molecules like vicinal diols (diols with hydroxyl groups on adjacent carbon atoms) or vicinal dihalides (dihalides with halogen atoms on adjacent carbon atoms) are involved in elimination reactions. • Isolated dienes : These have two double bonds, but they're separated by more than one single bond. Certain reactant structures can lead to either conjugated or isolated dienes. Reaction Examples 1. Dehydration of diols : o Example: When butane - 1,4 - diol undergoes dehydration, it forms 1,3 - butadiene . o Another example: pentane - 1,5 - diol can form 1,4 - pentadiene .

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    Study Guide 2. Dehydrohalogenation of dihalides : o Example: 1,4 - dibromobutane undergoes dehydrohalogenation to form 1,3 - butadiene . o Another example: 1,5 - dihalopentane can form 1,4 - pentadiene . Key Takeaway 1. Conjugated dienes are molecules with two alternating double bonds, separated by a single bond. 2. Dienes can be made through reactions like dehydration of diols or dehydrohalogenation of dihalides . 3. Vicinal diols and vicinal dihalides always form conjugated dienes when they undergo elimination reactions. 4. Depending on the structure of the reactants, the result can be either a conjugated or isolated diene.

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    Study Guide 2. 1,2 and 1,4 Additions in Dienes 1. How Dienes React in Addition Reactions Dienes are compounds that contain two double bonds . They can be: • Isolated dienes (double bonds are far apart) • Conjugated dienes (double bonds are next to each other, separated by one single bond) Both types of dienes undergo electrophilic addition reactions , just like alkenes. 2. Addition Reactions of Isolated Dienes In isolated dienes , each double bond behaves independently . • The reaction mechanism is the same as alkene addition • A carbocation intermediate forms • Addition follows Markovnikov or anti - Markovnikov rules Example: Addition of HBr to 1,4 - pentadiene • HBr adds across one double bond at a time • This produces two products : o A Markovnikov product o An anti - Markovnikov product If excess HBr is present, the second double bond can also react . No special behavior occurs here because the double bonds are not conjugated .

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    Study Guide 3. What Makes Conjugated Dienes Special? In conjugated dienes , the double bonds interact with each other through π - electron overlap . Because of this interaction: • The reaction does not stop at simple 1,2 - addition • A new type of product called 1,4 - addition is also formed 4. Addition of HBr to 1,3 - Butadiene When HBr reacts with 1,3 - butadiene , three products can form: 1. 1,2 - Addition (Markovnikov product) 2. 1,2 - Addition (anti - Markovnikov product) 3. 1,4 - Addition product (unique to conjugated dienes) The formation of the 1,4 - addition product is the key feature of conjugated dienes. 5. Why Does 1,4 - Addition Occur? The reason lies in the formation of a stable allylic carbocation . Allylic Carbocation An allylic carbocation forms when the positive charge is next to a double bond .

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    Study Guide • The positive charge is delocalized over two carbon atoms • This happens due to π - electron movement • This delocalization is called resonance Because of resonance: • The carbocation is much more stable • It exists as a resonance hybrid , not a single structure • Resonance stabilization is stronger than inductive stabilization The extra stability allows the bromide ion to attack two different positions , producing both 1,2 - and 1,4 - addition products . 6. Resonance and Stability Important points about resonance: • Resonance structures are not real molecules • The real molecule is a hybrid of all resonance forms • The hybrid has lower energy than any single structure • The energy difference is called: o Resonance energy o Conjugation energy o Delocalization energy More resonance structures = greater stability . Because it is resonance stabilized, the allylic carbocation is much more stable than an ordinary primary carbocation. Resonance stability always leads to a more stable state than inductive stability. The hybrid structure for this ion is

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    Study Guide This structure shows the π ‐ electron movement throughout the conjugated system, with a resulting delocation of the positive charge through the system. Understanding the allylic carbocation clarifies the mechanism for the addition to 1,3 ‐ butadiene. 7. Temperature Control: Kinetic vs Thermodynamic Products The product formed depends strongly on temperature . Low Temperature (Below 0°C) • Reaction is fast and irreversible • No equilibrium is established • Product formed faster dominates • Reaction is kinetically controlled • 1,2 - addition product is major High Temperature (Above 50°C) • Reaction becomes reversible • Equilibrium is established • Product with greater stability dominates • Reaction is thermodynamically controlled • 1,4 - addition product is major

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