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Organic Chemistry I - Structure and Properties of Alkenes

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    Study Guide Organic Chemistry I – Structure and Properties of Alkenes 1. Alkenes: Nomenclature (Naming Alkenes) Alkenes are named using the IUPAC system , just like alkanes. The rules are very similar, with a few important changes because alkenes contain a carbon – carbon double bond (C=C) . Let’s go step by step and make this easy to follow. Step 1: Find the Longest Chain with the Double Bond First, look for the longest continuous chain of carbon atoms that includes the double bond . • This chain becomes the parent chain . • Use the alkane name for the same number of carbons, but change – ane to – ene . Example: If the longest chain containing the double bond has five carbon atoms , the parent name is pentene . Step 2: Number the Carbon Chain Correctly Next, number the carbon atoms in the parent chain. • Always start numbering from the end closest to the double bond . • This ensures the double bond gets the lowest possible number . Why this matters: Numbering from the wrong end can give the double bond a higher number, which is not allowed in IUPAC naming. Step 3: Indicate the Position of the Double Bond Once the chain is numbered:

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    Study Guide • Identify the two carbons that form the double bond. • Use the lower number of those two carbons. • Place this number before the name of the alkene. Example: If the double bond is between carbon 2 and carbon 3, the compound is named 2 - pentene . Step 4: Name and Locate Substituents If there are any side groups (substituents) attached to the main chain: • Identify the substituent (like chloro, methyl, etc.). • Give its position number based on the numbering of the parent chain. • Write the substituent name before the parent alkene name. Example: A chlorine atom attached to carbon 5 of a six - carbon alkene gives the name 5 - chloro - 2 - hexene . Step 5: Add cis – trans Information (If Applicable) Some alkenes show geometrical (cis – trans) isomerism due to restricted rotation around the double bond. • cis : similar groups are on the same side of the double bond • trans : similar groups are on opposite sides If this information is known, it must be included in the name. Example: The complete name becomes cis - 5 - chloro - 2 - hexene .

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    Study Guide Key Takeaways • Alkenes are named using IUPAC rules , similar to alkanes. • The parent chain must be the longest chain containing the double bond . • The suffix – ene shows the presence of a double bond. • Number the chain from the end closest to the double bond . • Use the lowest number to show the position of the double bond. • Substituents are named and numbered before the parent alkene name. • cis – trans labels are added when the 3D arrangement around the double bond is known. 2. Alkenes: Physical Properties The physical properties of alkenes are very similar to those of alkanes. This is because both types of compounds are made mainly of carbon and hydrogen and have relatively nonpolar bonds. 2.1 Physical State of Alkenes Like alkanes, alkenes can exist in different physical states at room temperature: • Small alkenes are gases • Medium - sized alkenes are liquids • Large alkenes are solids The physical state mainly depends on the number of carbon atoms in the molecule.

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    Study Guide 2.2 Boiling Points of Alkenes Alkenes that have the same molecular formula but different structures (called isomeric alkenes ) usually have very similar boiling points . • Because their boiling points are so close, it is difficult to separate these isomers using simple boiling or distillation methods. 2.3 Dipole Moments in Substituted Alkenes When alkenes have atoms or groups other than hydrogen attached to the double - bonded carbons, they are called substituted alkenes . • These substitutions cause small differences in electron distribution . • As a result, substituted alkenes may show small dipole moments . These small dipole moments are especially useful for distinguishing cis and trans isomers . 2.4 Cis – Trans Isomers and Dipole Moment The polarity of cis and trans alkenes depends on how atoms or groups are arranged around the carbon – carbon double bond . • In cis isomers , similar groups are on the same side of the double bond. o Their bond dipoles add together , creating a net dipole moment. • In trans isomers , similar groups are on opposite sides. o Their bond dipoles cancel each other out , resulting in little or no dipole moment. Example: 2 - butene • cis - 2 - butene has a dipole moment of 0.33 debye (D) • trans - 2 - butene has a dipole moment of 0 D , because the electronic effects cancel out Important Point to Remember The effect of substitution on dipole moment must be analyzed separately for each molecule . It depends on:

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    Study Guide • The position of substituents • The electronegativity of the atoms or groups attached to the double bond Key Takeaways • Alkenes have physical properties similar to alkanes . • They can exist as gases, liquids, or solids at room temperature. • Isomeric alkenes usually have very similar boiling points . • Substituted alkenes may show small dipole moments . • Cis isomers often have a dipole moment due to uneven electron distribution. • Trans isomers may have zero dipole moment because bond dipoles cancel out. • cis - 2 - butene has a dipole moment of 0.33 D , while trans - 2 - butene has 0 D . 3. Alkenes: Preparations Alkenes are usually prepared by β - elimination reactions . In these reactions, two atoms are removed from adjacent carbon atoms , and as a result, a carbon – carbon double bond (C=C) is formed. 3.1 Common Methods to Prepare Alkenes Alkenes can be prepared in three main ways: 1. Dehydration of alcohols

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    Study Guide 2. Dehydrohalogenation of alkyl halides 3. Dehalogenation of vicinal dihalides Each of these methods involves elimination and leads to the formation of a double bond. Dehydration of Alcohols What Is Dehydration? In a dehydration reaction , an alcohol molecule loses a molecule of water (H ₂ O) . This reaction occurs when the alcohol is heated in the presence of a strong mineral acid , such as concentrated sulfuric acid (H ₂ SO ₄ ). As water is removed, a double bond forms between two adjacent carbon atoms . Example When ethanol is heated with concentrated sulfuric acid: • Ethanol → Ethene + Water 3.2 Mechanism of Alcohol Dehydration The dehydration of alcohols takes place in three main steps . Step 1: Protonation of the Alcohol The alcohol reacts with the acid in a simple acid – base reaction . The oxygen atom gains a proton and forms an oxonium ion , which has a positively charged oxygen atom .

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