Organic Chemistry I - Conjugated Dienes - Quick Guides | Chemistry

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Study GuideOrganic Chemistry I–Conjugated Dienes1.Preparation of Conjugated DienesConjugated dienesare a type of molecule that contains two double bonds in a row, with a singlebond 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 waterfrom the molecule.2.Dehydrohalogenation of dihalides(molecules with two halogen atoms)—this removes ahalogen atom.Whether the product is aconjugated dieneor anisolated dienedepends on how the originalmolecules were arranged.Conjugated vs. Isolated Dienes•Conjugated dienes: These have alternating single and double bonds, like inbutadiene.They often form when molecules likevicinal diols(diols with hydroxyl groups on adjacentcarbon atoms) orvicinal 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 onesingle bond. Certain reactant structures can lead to either conjugated or isolated dienes.Reaction Examples1.Dehydration of diols:oExample: Whenbutane-1,4-diolundergoes dehydration, it forms1,3-butadiene.oAnother example:pentane-1,5-diolcan form1,4-pentadiene.

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Study Guide2.Dehydrohalogenation of dihalides:oExample:1,4-dibromobutaneundergoes dehydrohalogenation to form1,3-butadiene.oAnother example:1,5-dihalopentanecan form1,4-pentadiene.Key Takeaway1.Conjugated dienesare molecules with two alternating double bonds, separated by a singlebond.2.Dienes can be made through reactions likedehydration of diolsordehydrohalogenationof dihalides.3.Vicinal diolsandvicinal dihalidesalways form conjugated dienes when they undergoelimination reactions.4.Depending on the structure of the reactants, the result can be either aconjugatedorisolateddiene.

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Study Guide2.1,2and 1,4Additions in Dienes1. How Dienes React in Addition ReactionsDienes are compounds that containtwo 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 undergoelectrophilic addition reactions, just like alkenes.2. Addition Reactions of Isolated DienesInisolated dienes, each double bond behavesindependently.•The reaction mechanism isthe same as alkene addition•Acarbocation intermediateforms•Addition followsMarkovnikov or anti-Markovnikov rulesExample: Addition of HBr to 1,4-pentadiene•HBr adds acrossone double bond at a time•This producestwo products:oAMarkovnikov productoAnanti-Markovnikov productIf excess HBr is present, thesecond double bond can also react.No special behavior occurs here because the double bonds arenot conjugated.

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Study Guide3. What Makes Conjugated Dienes Special?Inconjugated dienes, the double bonds interact with each other throughπ-electron overlap.Because of this interaction:•The reaction doesnot stop at simple 1,2-addition•A new type of product called1,4-additionis also formed4. Addition of HBr to 1,3-ButadieneWhenHBr 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 the1,4-addition productis the key feature of conjugated dienes.5. Why Does 1,4-Addition Occur?The reason lies in the formation of astable allylic carbocation.Allylic CarbocationAn allylic carbocation forms when the positive charge isnext to a double bond.

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Study Guide•The positive charge isdelocalizedover two carbon atoms•This happens due toπ-electron movement•This delocalization is calledresonanceBecause of resonance:•The carbocation ismuch more stable•It exists as aresonance hybrid, not a single structure•Resonance stabilization isstronger than inductive stabilizationThe extra stability allows the bromide ion to attacktwo different positions, producing both1,2-and1,4-addition products.6. Resonance and StabilityImportant points about resonance:•Resonance structures arenot real molecules•The real molecule is ahybridof all resonance forms•The hybrid haslower energythan any single structure•The energy difference is called:oResonance energyoConjugation energyoDelocalization energyMore resonance structures =greater stability.Because it is resonance stabilized, the allylic carbocation is much more stable than an ordinaryprimary 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 GuideThis structure shows theπ‐electron movement throughout the conjugated system, with a resultingdelocation 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 ProductsThe product formed depends strongly ontemperature.Low Temperature (Below 0°C)•Reaction isfast and irreversible•No equilibrium is established•Product formedfasterdominates•Reaction iskinetically controlled•1,2-addition productis majorHigh Temperature (Above 50°C)•Reaction becomesreversible•Equilibrium is established•Product withgreater stabilitydominates•Reaction isthermodynamically controlled•1,4-addition productis major

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