Organic Chemistry I - Structure and Properties of Alkenes - Quick Guides | Chemistry

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Study GuideOrganic Chemistry I–Structure and Properties ofAlkenes1. Alkenes: Nomenclature (Naming Alkenes)Alkenes are named using theIUPAC system, just like alkanes. The rules are very similar, with a fewimportant changes because alkenes contain acarbon–carbon double bond (C=C). Let’s go step bystep and make this easy to follow.Step 1: Find the Longest Chain with the Double BondFirst, look for thelongest continuous chain of carbon atomsthatincludes the double bond.•This chain becomes theparent chain.•Use the alkane name for the same number of carbons, but change–aneto–ene.Example:If the longest chain containing the double bond hasfive carbon atoms, the parent name ispentene.Step 2: Number the Carbon Chain CorrectlyNext, number the carbon atoms in the parent chain.•Always start numbering from theend closest to the double bond.•This ensures the double bond gets thelowest possible number.Why this matters:Numbering from the wrong end can give the double bond a higher number, which is not allowed inIUPAC naming.Step 3: Indicate the Position of the Double BondOnce the chain is numbered:

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Study Guide•Identify the two carbons that form the double bond.•Use thelower numberof those two carbons.•Place this numberbefore the nameof the alkene.Example:If the double bond is between carbon 2 and carbon 3, the compound is named2-pentene.Step 4: Name and Locate SubstituentsIf there are anyside groups (substituents)attached to the main chain:•Identify the substituent (like chloro, methyl, etc.).•Give itsposition numberbased on the numbering of the parent chain.•Write the substituent namebeforethe parent alkene name.Example:A chlorine atom attached to carbon 5 of a six-carbon alkene gives the name5-chloro-2-hexene.Step 5: Add cis–trans Information (If Applicable)Some alkenes showgeometrical (cis–trans) isomerismdue to restricted rotation around the doublebond.•cis: similar groups are on thesame sideof the double bond•trans: similar groups are onopposite sidesIf this information is known, it must be included in the name.Example:The complete name becomescis-5-chloro-2-hexene.

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Study GuideKey Takeaways•Alkenes are named usingIUPAC rules, similar to alkanes.•The parent chain must be thelongest chain containing the double bond.•The suffix–eneshows the presence of a double bond.•Number the chain from the endclosest to the double bond.•Use thelowest numberto show the position of the double bond.•Substituents are named and numberedbeforethe parent alkene name.•cis–trans labelsare added when the 3D arrangement around the double bond is known.2. Alkenes: Physical PropertiesThephysical properties of alkenesare very similar to those of alkanes. This is because both typesof compounds are made mainly of carbon and hydrogen and have relatively nonpolar bonds.2.1Physical State of AlkenesLike alkanes, alkenes can exist indifferent physical statesat room temperature:•Small alkenes aregases•Medium-sized alkenes areliquids•Large alkenes aresolidsThe physical state mainly depends on thenumber of carbon atomsin the molecule.

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Study Guide2.2Boiling Points of AlkenesAlkenes that have the same molecular formula but different structures (calledisomeric alkenes)usually havevery similar boiling points.•Because their boiling points are so close, it isdifficult to separatethese isomers usingsimple boiling or distillation methods.2.3Dipole Moments in Substituted AlkenesWhen alkenes have atoms or groups other than hydrogen attached to the double-bonded carbons,they are calledsubstituted alkenes.•These substitutions causesmall differences in electron distribution.•As a result, substituted alkenes may showsmall dipole moments.These small dipole moments are especially useful fordistinguishing cis and trans isomers.2.4Cis–Trans Isomers and Dipole MomentThe polarity of cis and trans alkenes depends on how atoms or groups are arranged around thecarbon–carbon double bond.•Incis isomers, similar groups are on the same side of the double bond.oTheir bond dipolesadd together, creating a net dipole moment.•Intrans isomers, similar groups are on opposite sides.oTheir bond dipolescancel each other out, resulting in little or no dipole moment.Example:2-butene•cis-2-butenehas a dipole moment of0.33 debye (D)•trans-2-butenehas a dipole moment of0 D, because the electronic effects cancel outImportant Point to RememberThe effect of substitution on dipole moment must beanalyzed separately for each molecule. Itdepends on:

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Study Guide•Thepositionof substituents•Theelectronegativityof the atoms or groups attached to the double bondKey Takeaways•Alkenes have physical propertiessimilar to alkanes.•They can exist asgases, liquids, or solidsat room temperature.•Isomeric alkenes usually havevery similar boiling points.•Substituted alkenes may showsmall dipole moments.•Cis isomersoften have a dipole moment due to uneven electron distribution.•Trans isomersmay have zero dipole moment because bond dipoles cancel out.•cis-2-butenehas a dipole moment of0.33 D, whiletrans-2-butenehas0 D.3. Alkenes: PreparationsAlkenes are usually prepared byβ-elimination reactions. In these reactions,two atoms areremoved from adjacent carbon atoms, and as a result, acarbon–carbon double bond (C=C)isformed.3.1Common Methods to Prepare AlkenesAlkenes can be prepared in three main ways:1.Dehydration of alcohols

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Study Guide2.Dehydrohalogenation of alkyl halides3.Dehalogenation of vicinal dihalidesEach of these methods involves elimination and leads to the formation of a double bond.Dehydration of AlcoholsWhat Is Dehydration?In adehydration reaction, an alcohol moleculeloses a molecule of water (H₂O).This reaction occurs when the alcohol isheated in the presence of a strong mineral acid, such asconcentrated sulfuric acid (H₂SO₄).As water is removed, adouble bond forms between two adjacent carbon atoms.ExampleWhen ethanol is heated with concentrated sulfuric acid:•Ethanol → Ethene + Water3.2Mechanism of Alcohol DehydrationThe dehydration of alcohols takes place inthree main steps.Step 1: Protonation of the AlcoholThe alcohol reacts with the acid in a simpleacid–base reaction.The oxygen atom gains a proton and forms anoxonium ion, which has apositively chargedoxygen atom.

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