Name: kekule’s benzene model and its limits
Uploaded: 2023-04-30T10:30:54.000Z
Duration: 1 min 7 s
Channel: JoVE

1865'te August Kekulé, benzen için ilk kesin yapıyı önerdi.
Benzenin, alternatif tek ve çift bağlara sahip altı karbon atomlu döngüsel bir yapıya sahip olduğunu öne sürdü. Her karbon atomu bir hidrojen atomuna bağlıdır.
Bu model, karbonun tetravalansını karşıladı ve tüm hidrojenleri eşdeğer hale getirdi.
Kekulé'nin modeli, çift bağların konumlarında farklılık gösteren iki 1,2-dibromobenzen formunu öngördü. Ancak, sadece bir 1,2-dibromobenzen formu izole edildi.
Kekulé, şimdi Kekulé yapıları olarak bilinen iki ayrılmaz benzen formu arasında hızlı bir denge önererek bunu savundu. Ancak artık böyle bir dengenin olmadığı bilinmektedir.
Ayrıca, bir ilave reaksiyona giren alkenlerin aksine, brom ile bir ikame reaksiyonuna giren benzenin olağandışı kimyasal davranışını açıklayamadı.

benzenin yapısı: kekulé modeli

Structure of Benzene: Kekul&eacute; Model

1865 yılında August Kekule, üç iddiaya dayanarak organik kimyanın yapısal teorisine göre benzenin yapısını önerdi: benzenin formülü C_6H_6'dır, benzenin tüm hidrojenleri eşdeğerdir ve tetravalansından dolayı her karbonun dört bağı olması gerekir.

Benzenin, her biri bir hidrojen atomuna bağlı, üç alternatif pi bağıyla altı karbon atomundan oluşan döngüsel bir yapıya sahip olduğunu öne sürdü.

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

Modeline göre, Şekil 2'de gösterildiği gibi 1,2-disübstitüe benzen türevleri için iki izomer bulunmalıdır; burada bir yapıda iki değiştirilmiş karbon, bir çift bağla, diğerinde ise tek bir bağla bağlanır.

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

1,2-disübstitüe benzen için yalnızca bir yapı bulunduğundan Kekule kavramının yanlış olduğu kanıtlandı. Bunu açıklamak için benzenin iki formunun hızlı dengede olduğunu ve iki bileşiği ayırmanın mümkün olmadığını belirtti.

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

Ancak böyle bir denge mevcut değildir ve 1,2-disübstitüe benzen türevleri için yalnızca bir yapı bulunur.

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

Benzen, bir halka ve üç pi bağından oluşan dört doymamışlık derecesine sahiptir. Ancak benzen, alkenler ve alkinler gibi diğer doymamış hidrokarbonlardan farklı olarak katılma reaksiyonlarına girmez. Örneğin etilen, etilen dibromür oluşturmak için brom ile bir katılma reaksiyonuna girer, ancak benzen böyle bir reaksiyona girmez.

<img alt="Figure5" src="/files/ftp_upload/12450/12450_PT_Figure5.svg" style="text-align: center; height: 120px;">

<img alt="Figure6" src="/files/ftp_upload/12450/12450_PT_Figure6.svg" style="text-align: center; height: 60px;">

Benzen, Lewis asidi ferrik bromür varlığında yer değiştirme reaksiyonu yoluyla bromobenzen oluşturmak üzere brom ile reaksiyona girer.

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

Kekule modelinin ana dezavantajı, alkenler ve alkinler gibi diğer doymamış hidrokarbonlarla karşılaştırıldığında benzenin bu olağandışı reaktivitesini açıklayamamasıdır.

Benzenin Yapısı: Kekulé Modeli

1865 yılında August Kekule, organik kimyanın yapısal teorisine göre benzenin yapısını üç temele dayanarak önerdi.

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kekule’s benzene model and its limits

Kekule’s benzene model describes benzene as a six-carbon ring with alternating pi bonds. August Kekule proposed this structure in 1865 from the formula C6H6, the idea that all benzene hydrogens are equivalent, and the rule that each carbon has four bonds because of tetravalency.

In this model, each carbon is attached to one hydrogen atom. It also predicts two isomers for 1,2-disubstituted benzene derivatives. In one form, the substituted carbons are linked by a double bond, and in the other they are linked by a single bond.

That prediction did not match experimental results. Only one structure is found for 1,2-disubstituted benzene derivatives. Kekule tried to explain this by saying the two forms of benzene stay in rapid equilibrium and cannot be separated, but no such equilibrium exists.

Benzene has four degrees of unsaturation, which means one ring and three pi bonds. Even so, it does not undergo addition reactions the way alkenes and alkynes do. Ethylene, for example, adds bromine to form ethylene dibromide, but benzene does not.

Instead, benzene reacts with bromine by substitution when ferric bromide, a Lewis acid, is present. The main weakness of Kekule’s model is that it does not explain this unusual reactivity compared with other unsaturated hydrocarbons.

Kekule’s Benzene Model and Its Limits

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Learn how Kekule proposed benzene’s ring structure, why it predicted two isomers, and why the model could not explain benzene’s unusual reactivity.

Kekule’s Benzene Model and Its Limits | JoVE K12

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Kekule’s Benzene Model and Its Limits

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