Topic 7 - Organic chemistry - organic reactions Flashcards
(25 cards)
What are alkenes?
- hydrocarbons with a double bond between two of the carbon atoms in their chain
- 2 fewer hydrogens compared with alkanes containing same number of carbon atoms
- general formula CnH2n
- part of a homologous series - all have same functional group C=C
Why are alkenes more reactive than alkanes?
C=C double bond can open up to make a single bond - allows the two carbon atoms to bond with other atoms
What are the first four alkenes?
- ethene - C2H4
- propene - C3H6
- butene - C4H8
- pentene - C5H10
Combustion of alkenes?
- in large amounts of oxygen alkenes combust completely to produce water and CO₂
- isnt enough exoygen in air for this - they trend to undergo incomplete combustion
- during incomplete combustion CO₂ and water are still produced - but also carbon and carbon monoxide
Why do alkenes burn with a smoky yellow flame?
- alkenes have higher carbon-to-hydrogen ratio than alkanes
- increased carbon content makes incomplete combustion more likely
- results in incomplete combustion - producing soot
- some of the carbon in the alkene did not fully oxidize to carbon dioxide
Compare energy released from combustion in alkanes and alkenes?
- due to incomplete combustion - alkenes release less energy per mole when they burn in combustion
- alkanes used as fuels - alkenes not
What are addition reactions?
when two molecules combine to form one larger molecule
How do alkenes react via addition reactions?
carbon-carbon double bond will open up to leave a single bond - new atom added to each carbon
How do alkenes react with hydrogen?
- addition of hydrogen known as hydrogenation
- hydrogen reacts with double-bonded carbons to open up double bond - form equivalent saturated alkane
- happens with nickel catalyst at 60°C
How do alkenes react halogens?
- alkenes react in addition reactions with halogens
- molecules formed are saturated - C=C bonds each become bonded to halogen atom
- bromine and ethane react together to form dibromoethane (colourless)
How do alkenes react with steam?
- when alkanes react with steam - water added across double bond and alcohol formed
- ethanol made by mixing ethene with steam and passing over catalyst
What are alcohols?
- another homologous series of organic compounds
- general formula CnH2n+1OH
- OH functional group
- names all end in suffix “ol”
What are the first four alcohols?
- Methanol CH3OH
- Ethanol C2H5OH
- Propanol C3H7OH
- Butanol C4H9OH
Properties of first four alcohols?
- flammable - undergo complete combustion in air to produce carbon dioxide and water
- are all soluble in water - solutions have neutral pH
- can be oxidised by reacting with oxygen to produce a carboxylic acid
Alcohols and carboxylic acids?
- different alcohols oxidide to form different carboxylic acids
- e.g. methanol is oxidised to for methanoic acid - ethanol is oxidised to form ethanoic acid
How are alcohols used as solvents?
- alchols like methanol and ethanol are used as solvents in industry
- they dissolve most things water can dissolve - can also dissolve things that water cannot dissolve e.g. hydrocarbons oils and fats
How are alcohols used as fuels?
- ethanol used in spirit burners + can be used as biofuel in cars
- it burns fairly cleanly and is not smelly
What happens when ethanol reactcs with sodium?
- hydrogen gas produced
- sodium ethoxide solution formed
How can ethanol be made from fermentation?
- ethanol is alcohol found in alcoholic drinks - usually made using fermentation
- fermentation uses enzyme in yeast to convert sugars to ethanol - ethanal produced is aqeous
- fermentation happens fastest at around 37°C in slightly acidic solution under anaerobic conditions
- enzyme in yeast works best to convert the sugar into alcohol in these conditions
What are carboxylic acids?
- homologous series
- all have COOH functional group
- names end in “-anoic acid”
First four carboxylic acids?
- methanoic acid (HCOOH)
- ethanoic acid (CH3COOH)
- propanoic acid (C2H5COOH)
- butanoic acid (C3H7COOH)
How do carboxylic acids react?
- react like any acid with carbonates to produce salt, water and carbon dioxide
- salts formed in these reactions end in “-anoate”
Why are carboxylic acids weak acids?
- do not ionise completely in water - only small proportion of their acid particles dissociate to form H+ ions
How can esters be made from carboxylic acids?
- esters have functional group “COO”
- carboxylic acids react with alcohols to form esters - an acid, usually sulfuric acid, is used as a catalyst
- alcohol + carboxylic acid –> ester + water
- e.g. ethanoic acid + ethanol –> ethyl ethanoate + water
